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Additional Topics in Trigonometry 6.1 Law of Sines 6.2 Law of Cosines 6.3 Vectors in the Plane 6.4 Vectors and Dot Products 6.5 Trigonometric Form of a Complex Number 6 In Mathematics Trigonometry is used to solve triangles, represent vectors, and to write trigonometric forms of complex numbers. In Real Life Luca Tettoni/Terra/Corbis Trigonometry is used to find areas, estimate heights, and represent vectors involving force, velocity, and other quantities. For instance, trigonometry and vectors can be used to find the tension in the tow lines as a loaded barge is being towed by two tugboats. (See Exercise 93, page 456.) IN CAREERS There are many careers that use trigonometry. Several are listed below. • Pilot Exercise 51, page 435 • Awning Designer Exercise 58, page 443 • Civil Engineer Exercise 55, page 443 • Landscaper Exercise 4, page 491 427 428 Chapter 6 Additional Topics in Trigonometry 6.1 LAW OF SINES What you should learn • Use the Law of Sines to solve oblique triangles (AAS or ASA). • Use the Law of Sines to solve oblique triangles (SSA). • Find the areas of oblique triangles. • Use the Law of Sines to model and solve real-life problems. Introduction In Chapter 4, you studied techniques for solving right triangles. In this section and the next, you will solve oblique triangles—triangles that have no right angles. As standard notation, the angles of a triangle are labeled A, B, and C, and their opposite sides are labeled a, b, and c, as shown in Figure 6.1. C Why you should learn it You can use the Law of Sines to solve real-life problems involving oblique triangles. For instance, in Exercise 53 on page 436, you can use the Law of Sines to determine the distance from a boat to the shoreline. a b A FIGURE B c 6.1 To solve an oblique triangle, you need to know the measure of at least one side and any two other measures of the triangle—either two sides, two angles, or one angle and one side. This breaks down into the following four cases. 1. Two angles and any side (AAS or ASA) © Owen Franken/Corbis 2. Two sides and an angle opposite one of them (SSA) 3. Three sides (SSS) 4. Two sides and their included angle (SAS) The first two cases can be solved using the Law of Sines, whereas the last two cases require the Law of Cosines (see Section 6.2). Law of Sines If ABC is a triangle with sides a, b, and c, then a b c . sin A sin B sin C C b C a h A c A is acute. h B b a A c B A is obtuse. The Law of Sines can also be written in the reciprocal form sin A sin B sin C . a b c For a proof of the Law of Sines, see Proofs in Mathematics on page 487. Section 6.1 Example 1 102° a Given Two Angles and One Side—AAS Solution The third angle of the triangle is 29° c FIGURE 429 For the triangle in Figure 6.2, C 102 , B 29 , and b 28 feet. Find the remaining angle and sides. C b = 28 ft A Law of Sines B A 180 B C 6.2 180 29 102 49 . By the Law of Sines, you have b c a . sin A sin B sin C Using b 28 produces When solving triangles, a careful sketch is useful as a quick test for the feasibility of an answer. Remember that the longest side lies opposite the largest angle, and the shortest side lies opposite the smallest angle. a 28 b sin A sin 49 sin B sin 29 c 28 b sin C sin 102 sin B sin 29 43.59 feet and 56.49 feet. Now try Exercise 5. Example 2 Given Two Angles and One Side—ASA A pole tilts toward the sun at an 8 angle from the vertical, and it casts a 22-foot shadow. The angle of elevation from the tip of the shadow to the top of the pole is 43 . How tall is the pole? Solution From Figure 6.3, note that A 43 and B 90 8 98 . So, the third angle is C C 180 A B 180 43 98 39 . b a By the Law of Sines, you have 8° c a . sin A sin C 43° B FIGURE 6.3 c = 22 ft A Because c 22 feet, the length of the pole is a c 22 sin A sin 43 sin C sin 39 23.84 feet. Now try Exercise 45. For practice, try reworking Example 2 for a pole that tilts away from the sun under the same conditions. 430 Chapter 6 Additional Topics in Trigonometry The Ambiguous Case (SSA) In Examples 1 and 2, you saw that two angles and one side determine a unique triangle. However, if two sides and one opposite angle are given, three possible situations can occur: (1) no such triangle exists, (2) one such triangle exists, or (3) two distinct triangles may satisfy the conditions. The Ambiguous Case (SSA) Consider a triangle in which you are given a, b, and A. h b sin A A is acute. A is acute. A is acute. A is acute. A is obtuse. Sketch b b a h h a b A A b a a h a h a b A A A is obtuse. a b A A Necessary condition a < h ah a b h < a < b a ! b a > b Triangles possible None One One Two None One Example 3 C For the triangle in Figure 6.4, a 22 inches, b 12 inches, and A 42 . Find the remaining side and angles. a = 22 in. b = 12 in. 42° A One solution: a b FIGURE 6.4 c Single-Solution Case—SSA B Solution By the Law of Sines, you have sin B sin A b a sin B b sin B 12 sin A a Reciprocal form sin 42 22 Multiply each side by b. 21.41 . B Substitute for A, a, and b. B is acute. Now, you can determine that C 180 42 21.41 116.59 . Then, the remaining side is c a sin C sin A c 22 a sin C sin 116.59 sin A sin 42 Now try Exercise 25. 29.40 inches. Section 6.1 Example 4 Law of Sines 431 No-Solution Case—SSA a = 15 b = 25 Show that there is no triangle for which a 15, b 25, and A 85 . h 85° A No solution: a < h FIGURE 6.5 Solution Begin by making the sketch shown in Figure 6.5. From this figure it appears that no triangle is formed. You can verify this using the Law of Sines. sin B sin A b a sin B b Reciprocal form sina A sin B 25 sin 85 15 Multiply each side by b. 1.660 > 1 This contradicts the fact that sin B ! 1. So, no triangle can be formed having sides a 15 and b 25 and an angle of A 85 . Now try Exercise 27. Example 5 Two-Solution Case—SSA Find two triangles for which a 12 meters, b 31 meters, and A 20.5 . Solution By the Law of Sines, you have sin B sin A b a sin B b Reciprocal form sin A sin 20.5 31 a 12 0.9047. There are two angles, B1 64.8 and B2 180 64.8 115.2 , between 0 and 180 whose sine is 0.9047. For B1 64.8 , you obtain C 180 20.5 64.8 94.7 c a 12 sin C sin 94.7 sin A sin 20.5 For B2 34.15 meters. 115.2 , you obtain C 180 20.5 115.2 44.3 c a 12 sin C sin 44.3 sin A sin 20.5 23.93 meters. The resulting triangles are shown in Figure 6.6. b = 31 m 20.5° A FIGURE b = 31 m a = 12 m 64.8° B1 6.6 Now try Exercise 29. A 20.5° 115.2° B2 a = 12 m 432 Chapter 6 Additional Topics in Trigonometry Area of an Oblique Triangle To see how to obtain the height of the obtuse triangle in Figure 6.7, notice the use of the reference angle 180 A and the difference formula for sine, as follows. h b sin180 A bsin 180 cos A The procedure used to prove the Law of Sines leads to a simple formula for the area of an oblique triangle. Referring to Figure 6.7, note that each triangle has a height of h b sin A. Consequently, the area of each triangle is Area 1 1 1 baseheight cb sin A bc sin A. 2 2 2 By similar arguments, you can develop the formulas 1 1 Area ab sin C ac sin B. 2 2 cos 180 sin A C C b0 " cos A 1 " sin A a b b sin A h A c A is acute. FIGURE 6.7 a b h B A c B A is obtuse. Area of an Oblique Triangle The area of any triangle is one-half the product of the lengths of two sides times the sine of their included angle. That is, 1 1 1 Area bc sin A ab sin C ac sin B. 2 2 2 Note that if angle A is 90 , the formula gives the area for a right triangle: Area 1 1 1 bc sin 90 bc baseheight. 2 2 2 sin 90 1 Similar results are obtained for angles C and B equal to 90 . Example 6 Finding the Area of a Triangular Lot Find the area of a triangular lot having two sides of lengths 90 meters and 52 meters and an included angle of 102 . Solution Consider a 90 meters, b 52 meters, and angle C 102 , as shown in Figure 6.8. Then, the area of the triangle is b = 52 m 102° C FIGURE 6.8 a = 90 m 1 1 Area ab sin C 9052sin 102 2 2 Now try Exercise 39. 2289 square meters. Section 6.1 Law of Sines 433 Application Example 7 N A W E S 52° An Application of the Law of Sines The course for a boat race starts at point A in Figure 6.9 and proceeds in the direction S 52 W to point B, then in the direction S 40 E to point C, and finally back to A. Point C lies 8 kilometers directly south of point A. Approximate the total distance of the race course. Solution B 8 km 40° Because lines BD and AC are parallel, it follows that BCA ! CBD. Consequently, triangle ABC has the measures shown in Figure 6.10. The measure of angle B is 180 52 40 88 . Using the Law of Sines, b c a . sin 52 sin 88 sin 40 C D FIGURE Because b 8, 6.9 A c 52° B b = 8 km a 40° a 8 sin 52 sin 88 6.308 c 8 sin 40 sin 88 5.145. and The total length of the course is approximately Length 19.453 kilometers. C FIGURE 8 6.308 5.145 6.10 Now try Exercise 49. CLASSROOM DISCUSSION Using the Law of Sines In this section, you have been using the Law of Sines to solve oblique triangles. Can the Law of Sines also be used to solve a right triangle? If so, write a short paragraph explaining how to use the Law of Sines to solve each triangle. Is there an easier way to solve these triangles? a. $AAS# b. $ASA# B B 50° C 50° c = 20 a = 10 A C A 434 Chapter 6 Additional Topics in Trigonometry EXERCISES 6.1 See www.CalcChat.com for worked-out solutions to odd-numbered exercises. VOCABULARY: Fill in the blanks. 1. An ________ triangle is a triangle that has no right angle. 2. For triangle ABC, the Law of Sines is given by c a ________ . sin A sin C 3. Two ________ and one ________ determine a unique triangle. 1 1 4. The area of an oblique triangle is given by 2 bc sin A 2ab sin C ________ . SKILLS AND APPLICATIONS In Exercises 5–24, use the Law of Sines to solve the triangle. Round your answers to two decimal places. 5. C b = 20 105° a 24. C 95.20 , a 35, c 50 45° c A 6. In Exercises 25–34, use the Law of Sines to solve (if possible) the triangle. If two solutions exist, find both. Round your answers to two decimal places. B C 25. A 110 , a 125, b 100 26. A 110 , a 125, b 200 27. A 76 , a 18, b 20 28. A 76 , a 34, b 21 29. A 58 , a 11.4, b 12.8 a b 35° 40° A B c = 10 7. C a = 3.5 b 25° A 35° B c 8. C b A 10° B c = 45 9. A 102.4 , C 16.7 , a 21.6 10. A 24.3 , C 54.6 , c 2.68 11. A 83 20#, C 54.6 , c 18.1 12. A 5 40#, B 8 15#, b 4.8 13. A 35 , B 65 , c 10 14. A 120 , B 45 , c 16 3 4 15. A 55 , B 42 , c 16. B 28 , C 104 , a 385 17. A 36 , a 8, b 5 18. A 60 , a 9, c 10 19. B 15 30#, a 4.5, 30. A 58 , a 4.5, b 12.8 31. A 120 , a b 25 32. A 120 , a 25, b 24 33. A 45 , a b 1 34. A 25 4#, a 135° 20. B 2 45#, b 6.2, c 5.8 21. A 145 , a 14, b 4 22. A 100 , a 125, c 10 23. A 110 15#, a 48, b 16 b 6.8 a 9.5, b 22 In Exercises 35–38, find values for b such that the triangle has (a) one solution, (b) two solutions, and (c) no solution. 35. A 36 , a 5 36. A 60 , a 10 37. A 10 , a 10.8 38. A 88 , a 315.6 In Exercises 39–44, find the area of the triangle having the indicated angle and sides. 39. C 120 , a 4, b 6 40. B 130 , a 62, c 20 41. A 43 45#, b 57, c 85 42. A 5 15#, b 4.5, c 22 43. B 72 30#, a 105, c 64 44. C 84 30#, a 16, b 20 Section 6.1 45. HEIGHT Because of prevailing winds, a tree grew so that it was leaning 4 from the vertical. At a point 40 meters from the tree, the angle of elevation to the top of the tree is 30 (see figure). Find the height h of the tree. 435 Law of Sines 49. BRIDGE DESIGN A bridge is to be built across a small lake from a gazebo to a dock (see figure). The bearing from the gazebo to the dock is S 41 W. From a tree 100 meters from the gazebo, the bearings to the gazebo and the dock are S 74 E and S 28 E, respectively. Find the distance from the gazebo to the dock. N Tree h 100 m 74° 94° Gazebo 40 m 41° 46. HEIGHT A flagpole at a right angle to the horizontal is located on a slope that makes an angle of 12 with the horizontal. The flagpole’s shadow is 16 meters long and points directly up the slope. The angle of elevation from the tip of the shadow to the sun is 20 . (a) Draw a triangle to represent the situation. Show the known quantities on the triangle and use a variable to indicate the height of the flagpole. (b) Write an equation that can be used to find the height of the flagpole. (c) Find the height of the flagpole. 47. ANGLE OF ELEVATION A 10-meter utility pole casts a 17-meter shadow directly down a slope when the angle of elevation of the sun is 42 (see figure). Find $, the angle of elevation of the ground. A B C 48. FLIGHT PATH A plane flies 500 kilometers with a bearing of 316 from Naples to Elgin (see figure). The plane then flies 720 kilometers from Elgin to Canton (Canton is due west of Naples). Find the bearing of the flight from Elgin to Canton. W N Elgin N E S 720 km Dock 50. RAILROAD TRACK DESIGN The circular arc of a railroad curve has a chord of length 3000 feet corresponding to a central angle of 40 . (a) Draw a diagram that visually represents the situation. Show the known quantities on the diagram and use the variables r and s to represent the radius of the arc and the length of the arc, respectively. (b) Find the radius r of the circular arc. (c) Find the length s of the circular arc. 51. GLIDE PATH A pilot has just started on the glide path for landing at an airport with a runway of length 9000 feet. The angles of depression from the plane to the ends of the runway are 17.5 and 18.8 . (a) Draw a diagram that visually represents the situation. (b) Find the air distance the plane must travel until touching down on the near end of the runway. 10 m 42° − θ m θ 17 500 km (c) Find the ground distance the plane must travel until touching down. (d) Find the altitude of the plane when the pilot begins the descent. 52. LOCATING A FIRE The bearing from the Pine Knob fire tower to the Colt Station fire tower is N 65 E, and the two towers are 30 kilometers apart. A fire spotted by rangers in each tower has a bearing of N 80 E from Pine Knob and S 70 E from Colt Station (see figure). Find the distance of the fire from each tower. 44° N W Canton E S 28° 30° 42° W Not drawn to scale Naples E Colt Station S 80° 65° 30 km Pine Knob 70° Fire Not drawn to scale 436 Chapter 6 Additional Topics in Trigonometry 53. DISTANCE A boat is sailing due east parallel to the shoreline at a speed of 10 miles per hour. At a given time, the bearing to the lighthouse is S 70 E, and 15 minutes later the bearing is S 63 E (see figure). The lighthouse is located at the shoreline. What is the distance from the boat to the shoreline? N 63° W 70° d E S 60. GRAPHICAL AND NUMERICAL ANALYSIS In the figure, & and % are positive angles. (a) Write & as a function of %. (b) Use a graphing utility to graph the function in part (a). Determine its domain and range. (c) Use the result of part (a) to write c as a function of %. (d) Use a graphing utility to graph the function in part (c). Determine its domain and range. (e) Complete the table. What can you infer? % 0.4 0.8 1.2 1.6 2.0 2.4 2.8 & 54. DISTANCE A family is traveling due west on a road that passes a famous landmark. At a given time the bearing to the landmark is N 62 W, and after the family travels 5 miles farther the bearing is N 38 W. What is the closest the family will come to the landmark while on the road? 55. ALTITUDE The angles of elevation to an airplane from two points A and B on level ground are 55 and 72 , respectively. The points A and B are 2.2 miles apart, and the airplane is east of both points in the same vertical plane. Find the altitude of the plane. 56. DISTANCE The angles of elevation $ and ' to an airplane from the airport control tower and from an observation post 2 miles away are being continuously monitored (see figure). Write an equation giving the distance d between the plane and observation post in terms of $ and '. 20 cm θ 2 8 cm γ 18 θ 9 30 cm β α c FIGURE FOR 60 FIGURE FOR 61 61. GRAPHICAL ANALYSIS (a) Write the area A of the shaded region in the figure as a function of $. (b) Use a graphing utility to graph the function. (c) Determine the domain of the function. Explain how the area of the region and the domain of the function would change if the eight-centimeter line segment were decreased in length. Airport control tower A c d Observation post θ B φ 2 mi Not drawn to scale EXPLORATION 62. CAPSTONE In the figure, a triangle is to be formed by drawing a line segment of length a from 4, 3 to the positive x-axis. For what value(s) of a can you form (a) one triangle, (b) two triangles, and (c) no triangles? Explain your reasoning. y TRUE OR FALSE? In Exercises 57–59, determine whether the statement is true or false. Justify your answer. 57. If a triangle contains an obtuse angle, then it must be oblique. 58. Two angles and one side of a triangle do not necessarily determine a unique triangle. 59. If three sides or three angles of an oblique triangle are known, then the triangle can be solved. (4, 3) 3 2 a 1 (0, 0) x 1 2 3 4 5 Section 6.2 Law of Cosines 437 6.2 LAW OF COSINES What you should learn Introduction • Use the Law of Cosines to solve oblique triangles (SSS or SAS). • Use the Law of Cosines to model and solve real-life problems. • Use Heron’s Area Formula to find the area of a triangle. Two cases remain in the list of conditions needed to solve an oblique triangle—SSS and SAS. If you are given three sides (SSS), or two sides and their included angle (SAS), none of the ratios in the Law of Sines would be complete. In such cases, you can use the Law of Cosines. Why you should learn it You can use the Law of Cosines to solve real-life problems involving oblique triangles. For instance, in Exercise 52 on page 443, you can use the Law of Cosines to approximate how far a baseball player has to run to make a catch. Law of Cosines Standard Form Alternative Form a 2 b2 c 2 2bc cos A cos A b2 c 2 a 2 2bc b2 a 2 c 2 2ac cos B cos B a 2 c 2 b2 2ac c 2 a 2 b2 2ab cos C cos C a 2 b2 c 2 2ab For a proof of the Law of Cosines, see Proofs in Mathematics on page 488. Example 1 Three Sides of a Triangle—SSS Find the three angles of the triangle in Figure 6.11. Daniel Bendjy/istockphoto.com B c = 14 ft a = 8 ft C A b = 19 ft FIGURE 6.11 Solution It is a good idea first to find the angle opposite the longest side—side b in this case. Using the alternative form of the Law of Cosines, you find that cos B a 2 c 2 b2 82 142 192 2ac 2814 0.45089. Because cos B is negative, you know that B is an obtuse angle given by B 116.80 . At this point, it is simpler to use the Law of Sines to determine A. sin A a sin B b 8 sin 116.80 19 0.37583 You know that A must be acute because B is obtuse, and a triangle can have, at most, one obtuse angle. So, A 22.08 and C 180 22.08 116.80 41.12 . Now try Exercise 5. 438 Chapter 6 Additional Topics in Trigonometry Do you see why it was wise to find the largest angle first in Example 1? Knowing the cosine of an angle, you can determine whether the angle is acute or obtuse. That is, cos $ > 0 for 0 < $ < 90 cos $ < 0 for 90 < $ < 180 . Acute Obtuse So, in Example 1, once you found that angle B was obtuse, you knew that angles A and C were both acute. If the largest angle is acute, the remaining two angles are acute also. Example 2 Two Sides and the Included Angle—SAS Find the remaining angles and side of the triangle in Figure 6.12. C b=9m a 25° c = 12 m A FIGURE B 6.12 Solution Use the Law of Cosines to find the unknown side a in the figure. When solving an oblique triangle given three sides, you use the alternative form of the Law of Cosines to solve for an angle. When solving an oblique triangle given two sides and their included angle, you use the standard form of the Law of Cosines to solve for an unknown. a 2 b2 c2 2bc cos A a 2 92 122 2912 cos 25 a2 29.2375 a 5.4072 Because a 5.4072 meters, you now know the ratio sin Aa and you can use the reciprocal form of the Law of Sines to solve for B. sin B sin A b a sin B b 9 sina A sin 25 5.4072 0.7034 There are two angles between 0 and 180 whose sine is 0.7034, B1 B2 180 44.7 135.3 . For B1 C1 For B2 C2 44.7 and 44.7 , 180 25 44.7 110.3 . 135.3 , 180 25 135.3 19.7 . Because side c is the longest side of the triangle, C must be the largest angle of the triangle. So, B 44.7 and C 110.3 . Now try Exercise 7. Section 6.2 439 Law of Cosines Applications Example 3 60 ft The pitcher’s mound on a women’s softball field is 43 feet from home plate and the distance between the bases is 60 feet, as shown in Figure 6.13. (The pitcher’s mound is not halfway between home plate and second base.) How far is the pitcher’s mound from first base? 60 ft Solution h P F f = 43 ft 45° 60 ft 6.13 In triangle HPF, H 45 (line HP bisects the right angle at H), f 43, and p 60. Using the Law of Cosines for this SAS case, you have h2 f 2 p 2 2fp cos H 432 602 24360 cos 45 H FIGURE p = 60 ft An Application of the Law of Cosines 1800.3. So, the approximate distance from the pitcher’s mound to first base is h 1800.3 42.43 feet. Now try Exercise 43. Example 4 An Application of the Law of Cosines A ship travels 60 miles due east, then adjusts its course northward, as shown in Figure 6.14. After traveling 80 miles in that direction, the ship is 139 miles from its point of departure. Describe the bearing from point B to point C. N W E B A FIGURE C i b = 139 m S 0 mi a=8 c = 60 mi 6.14 Solution You have a 80, b 139, and c 60. So, using the alternative form of the Law of Cosines, you have cos B a 2 c 2 b2 2ac 802 602 1392 28060 0.97094. So, B arccos0.97094 from point B to point C is 166.15 , and thus the bearing measured from due north 166.15 90 76.15 , or N 76.15 E. Now try Exercise 49. 440 Chapter 6 Additional Topics in Trigonometry Heron’s Area Formula HISTORICAL NOTE Heron of Alexandria (c. 100 B.C.) was a Greek geometer and inventor. His works describe how to find the areas of triangles, quadrilaterals, regular polygons having 3 to 12 sides, and circles as well as the surface areas and volumes of three-dimensional objects. The Law of Cosines can be used to establish the following formula for the area of a triangle. This formula is called Heron’s Area Formula after the Greek mathematician Heron (c. 100 B.C.). Heron’s Area Formula Given any triangle with sides of lengths a, b, and c, the area of the triangle is Area ss as bs c where s a b c2. For a proof of Heron’s Area Formula, see Proofs in Mathematics on page 489. Example 5 Using Heron’s Area Formula Find the area of a triangle having sides of lengths a 43 meters, b 53 meters, and c 72 meters. Solution Because s a b c2 1682 84, Heron’s Area Formula yields Area ss as bs c 84413112 1131.89 square meters. Now try Exercise 59. You have now studied three different formulas for the area of a triangle. 1 Standard Formula: Area 2bh Oblique Triangle: 1 1 Area 12bc sin A 2 ab sin C 2ac sin B Heron’s Area Formula: Area ss as bs c CLASSROOM DISCUSSION The Area of a Triangle Use the most appropriate formula to find the area of each triangle below. Show your work and give your reasons for choosing each formula. a. b. 3 ft 2 ft 2 ft 50° 4 ft c. 4 ft d. 2 ft 4 ft 4 ft 3 ft 5 ft Section 6.2 EXERCISES 6.2 441 Law of Cosines See www.CalcChat.com for worked-out solutions to odd-numbered exercises. VOCABULARY: Fill in the blanks. 1. If you are given three sides of a triangle, you would use the Law of ________ to find the three angles of the triangle. 2. If you are given two angles and any side of a triangle, you would use the Law of ________ to solve the triangle. 3. The standard form of the Law of Cosines for cos B a2 c2 b2 is ________ . 2ac 4. The Law of Cosines can be used to establish a formula for finding the area of a triangle called ________ ________ Formula. SKILLS AND APPLICATIONS In Exercises 5–20, use the Law of Cosines to solve the triangle. Round your answers to two decimal places. 5. 6. C C a = 10 b = 12 a=7 b=3 A A 7. B c=8 B c = 16 8. C b = 15 a 30° A c = 30 B c B 9. a 11, b 15, c 21 10. a 55, b 25, c 72 11. a 75.4, b 52, c 52 14. A 48 , b 3, c 14 15. B 10 35#, a 40, c 30 16. B 75 20#, a 6.2, c 9.5 17. B 125 40# , a 37, c 37 18. C 15 15#, a 7.45, b 2.15 7 4 19. C 43 , a 9, b 9 3 3 4 In Exercises 21–26, complete the table by solving the parallelogram shown in the figure. (The lengths of the diagonals are given by c and d.) c φ a d θ b 22. 25 23. 10 35 14 24. 40 60 25 c d $ 45 20 80 25 20 50 35 ' 120 In Exercises 27–32, determine whether the Law of Sines or the Law of Cosines is needed to solve the triangle. Then solve the triangle. 27. a 8, 12. a 1.42, b 0.75, c 1.25 13. A 120 , b 6, c 7 20. C 101 , a 8, b 8 a=9 105° A b 5 25. 15 26. C b = 4.5 a 21. c 5, B 40 28. a 10, b 12, C 70 29. A 24 , a 4, b 18 30. a 11, b 13, c 7 31. A 42 , B 35 , c 1.2 32. a 160, B 12 , C 7 In Exercises 33–40, use Heron’s Area Formula to find the area of the triangle. 33. a 8, b 12, c 17 34. a 33, b 36, c 25 35. a 2.5, b 10.2, c 9 36. a 75.4, b 52, c 52 37. a 12.32, b 8.46, c 15.05 38. a 3.05, b 0.75, c 2.45 1 3 39. a 1, b 2, c 4 5 3 3 40. a 5, b 8, c 8 442 Chapter 6 Additional Topics in Trigonometry 41. NAVIGATION A boat race runs along a triangular course marked by buoys A, B, and C. The race starts with the boats headed west for 3700 meters. The other two sides of the course lie to the north of the first side, and their lengths are 1700 meters and 3000 meters. Draw a figure that gives a visual representation of the situation, and find the bearings for the last two legs of the race. 42. NAVIGATION A plane flies 810 miles from Franklin to Centerville with a bearing of 75 . Then it flies 648 miles from Centerville to Rosemount with a bearing of 32 . Draw a figure that visually represents the situation, and find the straight-line distance and bearing from Franklin to Rosemount. 43. SURVEYING To approximate the length of a marsh, a surveyor walks 250 meters from point A to point B, then turns 75 and walks 220 meters to point C (see figure). Approximate the length AC of the marsh. B 48. LENGTH A 100-foot vertical tower is to be erected on the side of a hill that makes a 6 angle with the horizontal (see figure). Find the length of each of the two guy wires that will be anchored 75 feet uphill and downhill from the base of the tower. 100 ft 6° 75 ft 75 ft 49. NAVIGATION On a map, Orlando is 178 millimeters due south of Niagara Falls, Denver is 273 millimeters from Orlando, and Denver is 235 millimeters from Niagara Falls (see figure). 75° 220 m 250 m 235 mm C A Niagara Falls Denver 178 mm 273 mm 44. SURVEYING A triangular parcel of land has 115 meters of frontage, and the other boundaries have lengths of 76 meters and 92 meters. What angles does the frontage make with the two other boundaries? 45. SURVEYING A triangular parcel of ground has sides of lengths 725 feet, 650 feet, and 575 feet. Find the measure of the largest angle. 46. STREETLIGHT DESIGN Determine the angle $ in the design of the streetlight shown in the figure. Orlando (a) Find the bearing of Denver from Orlando. (b) Find the bearing of Denver from Niagara Falls. 50. NAVIGATION On a map, Minneapolis is 165 millimeters due west of Albany, Phoenix is 216 millimeters from Minneapolis, and Phoenix is 368 millimeters from Albany (see figure). Minneapolis 165 mm 3 Albany 216 mm 368 mm θ 2 4 12 Phoenix (a) Find the bearing of Minneapolis from Phoenix. 47. DISTANCE Two ships leave a port at 9 A.M. One travels at a bearing of N 53 W at 12 miles per hour, and the other travels at a bearing of S 67 W at 16 miles per hour. Approximate how far apart they are at noon that day. (b) Find the bearing of Albany from Phoenix. 51. BASEBALL On a baseball diamond with 90-foot sides, the pitcher’s mound is 60.5 feet from home plate. How far is it from the pitcher’s mound to third base? Section 6.2 52. BASEBALL The baseball player in center field is playing approximately 330 feet from the television camera that is behind home plate. A batter hits a fly ball that goes to the wall 420 feet from the camera (see figure). The camera turns 8 to follow the play. Approximately how far does the center fielder have to run to make the catch? 443 Law of Cosines 56. ENGINE DESIGN An engine has a seven-inch connecting rod fastened to a crank (see figure). (a) Use the Law of Cosines to write an equation giving the relationship between x and $. (b) Write x as a function of $. (Select the sign that yields positive values of x.) (c) Use a graphing utility to graph the function in part (b). (d) Use the graph in part (c) to determine the maximum distance the piston moves in one cycle. 330 ft 8° 1.5 in. 420 ft 3 in. 7 in. s θ θ x 53. AIRCRAFT TRACKING To determine the distance between two aircraft, a tracking station continuously determines the distance to each aircraft and the angle A between them (see figure). Determine the distance a between the planes when A 42 , b 35 miles, and c 20 miles. a C d 4 in. 6 in. FIGURE FOR 56 FIGURE FOR 57 57. PAPER MANUFACTURING In a process with continuous paper, the paper passes across three rollers of radii 3 inches, 4 inches, and 6 inches (see figure). The centers of the three-inch and six-inch rollers are d inches apart, and the length of the arc in contact with the paper on the four-inch roller is s inches. Complete the table. B d (inches) b c 9 10 12 13 14 15 16 $ (degrees) s (inches) A 54. AIRCRAFT TRACKING Use the figure for Exercise 53 to determine the distance a between the planes when A 11 , b 20 miles, and c 20 miles. 55. TRUSSES Q is the midpoint of the line segment PR in the truss rafter shown in the figure. What are the lengths of the line segments PQ, QS, and RS ? 58. AWNING DESIGN A retractable awning above a patio door lowers at an angle of 50 from the exterior wall at a height of 10 feet above the ground (see figure). No direct sunlight is to enter the door when the angle of elevation of the sun is greater than 70 . What is the length x of the awning? R x Q P Suns rays 10 10 ft S 8 50° 8 8 8 70° 59. GEOMETRY The lengths of the sides of a triangular parcel of land are approximately 200 feet, 500 feet, and 600 feet. Approximate the area of the parcel. 444 Chapter 6 Additional Topics in Trigonometry 60. GEOMETRY A parking lot has the shape of a parallelogram (see figure). The lengths of two adjacent sides are 70 meters and 100 meters. The angle between the two sides is 70 . What is the area of the parking lot? a b c . sin A sin B sin C s as bs c (b) Prove that r . s (a) Prove that 2R CIRCUMSCRIBED AND INSCRIBED CIRCLES In Exercises 68 and 69, use the results of Exercise 67. 70 m 68. Given a triangle with a 25, b 55, and c 72, find the areas of (a) the triangle, (b) the circumscribed circle, and (c) the inscribed circle. 70° 100 m 61. GEOMETRY You want to buy a triangular lot measuring 510 yards by 840 yards by 1120 yards. The price of the land is $2000 per acre. How much does the land cost? (Hint: 1 acre 4840 square yards) 62. GEOMETRY You want to buy a triangular lot measuring 1350 feet by 1860 feet by 2490 feet. The price of the land is $2200 per acre. How much does the land cost? (Hint: 1 acre 43,560 square feet) EXPLORATION TRUE OR FALSE? In Exercises 63 and 64, determine whether the statement is true or false. Justify your answer. 63. In Heron’s Area Formula, s is the average of the lengths of the three sides of the triangle. 64. In addition to SSS and SAS, the Law of Cosines can be used to solve triangles with SSA conditions. 65. WRITING A triangle has side lengths of 10 centimeters, 16 centimeters, and 5 centimeters. Can the Law of Cosines be used to solve the triangle? Explain. 66. WRITING Given a triangle with b 47 meters, A 87 , and C 110 , can the Law of Cosines be used to solve the triangle? Explain. 67. CIRCUMSCRIBED AND INSCRIBED CIRCLES Let R and r be the radii of the circumscribed and inscribed circles of a triangle ABC, respectively (see figure), and let abc . s 2 69. Find the length of the largest circular running track that can be built on a triangular piece of property with sides of lengths 200 feet, 250 feet, and 325 feet. 70. THINK ABOUT IT What familiar formula do you obtain when you use the third form of the Law of Cosines c2 a2 b2 2ab cos C, and you let C 90 ? What is the relationship between the Law of Cosines and this formula? 71. THINK ABOUT IT In Example 2, suppose A 115 . After solving for a, which angle would you solve for next, B or C? Are there two possible solutions for that angle? If so, how can you determine which angle is the correct solution? 72. WRITING Describe how the Law of Cosines can be used to solve the ambiguous case of the oblique triangle ABC, where a 12 feet, b 30 feet, and A 20 . Is the result the same as when the Law of Sines is used to solve the triangle? Describe the advantages and the disadvantages of each method. 73. WRITING In Exercise 72, the Law of Cosines was used to solve a triangle in the two-solution case of SSA. Can the Law of Cosines be used to solve the no-solution and single-solution cases of SSA? Explain. 74. CAPSTONE Determine whether the Law of Sines or the Law of Cosines is needed to solve the triangle. (a) A, C, and a (c) b, c, and A (b) a, c, and C (d) A, B, and c (e) b, c, and C (f) a, b, and c A 75. PROOF b C r a R c Use the Law of Cosines to prove that abc 1 bc 1 cos A 2 2 " a b c . 2 B 76. PROOF Use the Law of Cosines to prove that 1 abc bc 1 cos A 2 2 " abc . 2 Section 6.3 445 Vectors in the Plane 6.3 VECTORS IN THE PLANE What you should learn • Represent vectors as directed line segments. • Write the component forms of vectors. • Perform basic vector operations and represent them graphically. • Write vectors as linear combinations of unit vectors. • Find the direction angles of vectors. • Use vectors to model and solve real-life problems. Introduction Quantities such as force and velocity involve both magnitude and direction and cannot be completely characterized by a single real number. To represent such a quantity, you can use a directed line segment, as shown in Figure 6.15. The directed line segment PQ has initial point P and terminal point Q. Its magnitude (or length) is denoted by PQ and can be found using the Distance Formula. \ \ Terminal point PQ P Bill Bachman/Photo Researchers, Inc. Initial point FIGURE Why you should learn it You can use vectors to model and solve real-life problems involving magnitude and direction. For instance, in Exercise 102 on page 457, you can use vectors to determine the true direction of a commercial jet. Q 6.15 FIGURE 6.16 Two directed line segments that have the same magnitude and direction are equivalent. For example, the directed line segments in Figure 6.16 are all equivalent. The set of all directed line segments that are equivalent to the directed line segment PQ is a vector v in the plane, written v PQ . Vectors are denoted by lowercase, boldface letters such as u, v, and w. \ \ Example 1 Vector Representation by Directed Line Segments Let u be represented by the directed line segment from P0, 0 to Q3, 2, and let v be represented by the directed line segment from R1, 2 to S4, 4, as shown in Figure 6.17. Show that u and v are equivalent. y 5 (4, 4) 4 3 (1, 2) 2 R 1 v u P (0, 0) 1 FIGURE 6.17 S (3, 2) Q x 2 3 4 Solution \ \ From the Distance Formula, it follows that PQ and RS have the same magnitude. PQ 3 0 2 2 0 2 13 \ RS 4 1 2 4 2 2 13 \ Moreover, both line segments have the same direction because they are both directed toward the upper right on lines having a slope of 42 20 2 . 41 30 3 \ \ Because PQ and RS have the same magnitude and direction, u and v are equivalent. Now try Exercise 11. 446 Chapter 6 Additional Topics in Trigonometry Component Form of a Vector The directed line segment whose initial point is the origin is often the most convenient representative of a set of equivalent directed line segments. This representative of the vector v is in standard position. A vector whose initial point is the origin 0, 0 can be uniquely represented by the coordinates of its terminal point v1, v2. This is the component form of a vector v, written as v !v1, v2". The coordinates v1 and v2 are the components of v. If both the initial point and the terminal point lie at the origin, v is the zero vector and is denoted by 0 !0, 0". Component Form of a Vector The component form of the vector with initial point P p1, p2 and terminal point Qq1, q2 is given by \ PQ !q1 p1, q2 p2" !v1, v2" v. The magnitude (or length) of v is given by T E C H N O LO G Y You can graph vectors with a graphing utility by graphing directed line segments. Consult the user’s guide for your graphing utility for specific instructions. v q1 p12 q2 p2 2 v12 v22. If v 1, v is a unit vector. Moreover, v 0 if and only if v is the zero vector 0. Two vectors u !u1, u2" and v !v1, v2" are equal if and only if u1 v1 and u2 v2. For instance, in Example 1, the vector u from P0, 0 to Q3, 2 is u PQ !3 0, 2 0" !3, 2", and the vector v from R1, 2 to S4, 4 is v RS !4 1, 4 2" !3, 2". \ \ Example 2 Finding the Component Form of a Vector Find the component form and magnitude of the vector v that has initial point 4, 7 and terminal point 1, 5. Algebraic Solution Graphical Solution Let Use centimeter graph paper to plot the points P4, 7 and Q1, 5. Carefully sketch the vector v. Use the sketch to find the components of v !v1, v2". Then use a centimeter ruler to find the magnitude of v. P4, 7 p1, p2 and 11 12 13 Q1, 5 q1, q2. 9 10 Then, the components of v !v1, v2" are 7 8 v1 q1 p1 1 4 5 4 5 6 v2 q2 p2 5 7 12. 2 3 So, v !5, 12" and the magnitude of v is 1 cm v 52 122 169 13. FIGURE 6.18 Figure 6.18 shows that the components of v are v1 5 and v2 12, so v !5, 12". Figure 6.18 also shows that the magnitude of v is v 13. Now try Exercise 19. Section 6.3 447 Vectors in the Plane Vector Operations 1 2 v v 2v −v − 32 v The two basic vector operations are scalar multiplication and vector addition. In operations with vectors, numbers are usually referred to as scalars. In this text, scalars will always be real numbers. Geometrically, the product of a vector v and a scalar k is the vector that is k times as long as v. If k is positive, kv has the same direction as v, and if k is negative, kv has the direction opposite that of v, as shown in Figure 6.19. To add two vectors u and v geometrically, first position them (without changing their lengths or directions) so that the initial point of the second vector v coincides with the terminal point of the first vector u. The sum u v is the vector formed by joining the initial point of the first vector u with the terminal point of the second vector v, as shown in Figure 6.20. This technique is called the parallelogram law for vector addition because the vector u v, often called the resultant of vector addition, is the diagonal of a parallelogram having adjacent sides u and v. FIGURE 6.19 y y v u+ v u u v x FIGURE x 6.20 Definitions of Vector Addition and Scalar Multiplication Let u !u1, u2" and v !v1, v2" be vectors and let k be a scalar (a real number). Then the sum of u and v is the vector u v !u1 v1, u2 v2" Sum and the scalar multiple of k times u is the vector y ku k!u1, u2" !ku1, ku2". Scalar multiple The negative of v !v1, v2" is v 1v −v u−v !v1, v2" and the difference of u and v is u u v u v v !u1 v1, u2 v2". u + (−v) x u v u v FIGURE 6.21 Negative Add v. See Figure 6.21. Difference To represent u v geometrically, you can use directed line segments with the same initial point. The difference u v is the vector from the terminal point of v to the terminal point of u, which is equal to u v, as shown in Figure 6.21. 448 Chapter 6 Additional Topics in Trigonometry The component definitions of vector addition and scalar multiplication are illustrated in Example 3. In this example, notice that each of the vector operations can be interpreted geometrically. Example 3 Vector Operations Let v !2, 5" and w !3, 4", and find each of the following vectors. b. w v a. 2v c. v 2w Solution a. Because v !2, 5", you have 2v 2!2, 5" !22, 25" !4, 10". A sketch of 2v is shown in Figure 6.22. b. The difference of w and v is w v !3, 4" !2, 5" !3 2, 4 5" !5, 1". A sketch of w v is shown in Figure 6.23. Note that the figure shows the vector difference w v as the sum w v. c. The sum of v and 2w is v 2w !2, 5" 2!3, 4" !2, 5" !23, 24" !2, 5" !6, 8" !2 6, 5 8" !4, 13". A sketch of v 2w is shown in Figure 6.24. y (− 4, 10) y 10 y (3, 4) 4 (4, 13) 14 12 8 3 10 2v 6 2 4 1 w 8 −v (−2, 5) v x x −8 FIGURE −6 6.22 −4 −2 2 −1 FIGURE v + 2w (−2, 5) w−v 3 4 (5, −1) 6.23 Now try Exercise 31. 2w v 5 x −6 −4 −2 FIGURE 6.24 2 4 6 8 Section 6.3 Vectors in the Plane 449 Vector addition and scalar multiplication share many of the properties of ordinary arithmetic. Properties of Vector Addition and Scalar Multiplication Let u, v, and w be vectors and let c and d be scalars. Then the following properties are true. 1. u v v u 2. u v w u v w 3. u 0 u 4. u u 0 5. cd u cd u 6. c du cu d u 7. cu v cu cv 8. 1u u, 9. cv c 0u 0 v Property 9 can be stated as follows: the magnitude of the vector cv is the absolute value of c times the magnitude of v. HISTORICAL NOTE Unit Vectors The Granger Collection In many applications of vectors, it is useful to find a unit vector that has the same direction as a given nonzero vector v. To do this, you can divide v by its magnitude to obtain William Rowan Hamilton (1805–1865), an Irish mathematician, did some of the earliest work with vectors. Hamilton spent many years developing a system of vector-like quantities called quaternions. Although Hamilton was convinced of the benefits of quaternions, the operations he defined did not produce good models for physical phenomena. It was not until the latter half of the nineteenth century that the Scottish physicist James Maxwell (1831–1879) restructured Hamilton’s quaternions in a form useful for representing physical quantities such as force, velocity, and acceleration. u unit vector v v v v. 1 Unit vector in direction of v Note that u is a scalar multiple of v. The vector u has a magnitude of 1 and the same direction as v. The vector u is called a unit vector in the direction of v. Example 4 Finding a Unit Vector Find a unit vector in the direction of v !2, 5" and verify that the result has a magnitude of 1. Solution The unit vector in the direction of v is v !2, 5" 2 2 52 v 1 29 !2, 5" $229, 529%. This vector has a magnitude of 1 because 229 529 294 2925 2929 1. 2 2 Now try Exercise 41. 450 Chapter 6 Additional Topics in Trigonometry The unit vectors !1, 0" and !0, 1" are called the standard unit vectors and are denoted by y i !1, 0" 2 j !0, 1" as shown in Figure 6.25. (Note that the lowercase letter i is written in boldface to distinguish it from the imaginary number i 1.) These vectors can be used to represent any vector v !v1, v2", as follows. j = 〈0, 1〉 1 and v !v1, v2" v1!1, 0" v2!0, 1" i = 〈1, 0〉 x 1 FIGURE 2 v1i v2 j The scalars v1 and v2 are called the horizontal and vertical components of v, respectively. The vector sum 6.25 v1i v2 j is called a linear combination of the vectors i and j. Any vector in the plane can be written as a linear combination of the standard unit vectors i and j. Example 5 y Let u be the vector with initial point 2, 5 and terminal point 1, 3. Write u as a linear combination of the standard unit vectors i and j. 8 6 (−1, 3) Solution 4 Begin by writing the component form of the vector u. x −8 −6 −4 −2 2 −2 4 u −6 6 u !1 2, 3 5" !3, 8" 3i 8j −4 FIGURE Writing a Linear Combination of Unit Vectors (2, −5) This result is shown graphically in Figure 6.26. Now try Exercise 53. 6.26 Example 6 Vector Operations Let u 3i 8j and let v 2i j. Find 2u 3v. Solution You could solve this problem by converting u and v to component form. This, however, is not necessary. It is just as easy to perform the operations in unit vector form. 2u 3v 23i 8j 32i j 6i 16j 6i 3j 12i 19j Now try Exercise 59. Section 6.3 y 451 Direction Angles 1 If u is a unit vector such that is the angle (measured counterclockwise) from the positive x-axis to u, the terminal point of u lies on the unit circle and you have (x, y) u u !x, y" !cos , sin " cos i sin j y = sin θ θ −1 Vectors in the Plane x x = cos θ 1 −1 as shown in Figure 6.27. The angle is the direction angle of the vector u. Suppose that u is a unit vector with direction angle . If v a i bj is any vector that makes an angle with the positive x-axis, it has the same direction as u and you can write v v !cos , sin " v cos i v sin j. u 1 FIGURE 6.27 Because v a i bj v cos i v sin j, it follows that the direction angle for v is determined from tan sin cos Quotient identity v sin v cos Multiply numerator and denominator by v . b . a Simplify. Example 7 Finding Direction Angles of Vectors Find the direction angle of each vector. y a. u 3i 3j (3, 3) 3 2 b. v 3i 4j Solution u a. The direction angle is 1 θ = 45° FIGURE 2 tan x 1 3 6.28 So, b 3 1. a 3 45!, as shown in Figure 6.28. b. The direction angle is y 1 306.87° x −1 −1 1 2 v −2 FIGURE 3 4 b 4 . a 3 Moreover, because v 3i 4j lies in Quadrant IV, reference angle is (3, −4) 6.29 lies in Quadrant IV and its 4 3 & 53.13! 53.13!. arctan −3 −4 tan So, it follows that & 360! 53.13! 306.87!, as shown in Figure 6.29. Now try Exercise 63. 452 Chapter 6 Additional Topics in Trigonometry Applications of Vectors Example 8 y Find the component form of the vector that represents the velocity of an airplane descending at a speed of 150 miles per hour at an angle 20! below the horizontal, as shown in Figure 6.30. 200° x − 150 − 50 Finding the Component Form of a Vector Solution The velocity vector v has a magnitude of 150 and a direction angle of 150 −50 200!. v v cos i v sin j 150cos 200!i 150sin 200!j − 100 & 1500.9397i 1500.3420j & 140.96i 51.30j FIGURE !140.96, 51.30" 6.30 You can check that v has a magnitude of 150, as follows. v & 140.962 51.302 & 19,869.72 2631.69 22,501.41 & 150 Now try Exercise 83. Example 9 Using Vectors to Determine Weight A force of 600 pounds is required to pull a boat and trailer up a ramp inclined at 15! from the horizontal. Find the combined weight of the boat and trailer. Solution Based on Figure 6.31, you can make the following observations. \ BA force of gravity combined weight of boat and trailer \ BC force against ramp \ AC force required to move boat up ramp 600 pounds B W 15° D 15° A C By construction, triangles BWD and ABC are similar. Therefore, angle ABC is 15!. So, in triangle ABC you have \ sin 15! FIGURE 6.31 sin 15! \ BA AC BA \ 600 \ BA 600 sin 15! \ BA & 2318. Consequently, the combined weight is approximately 2318 pounds. (In Figure 6.31, note that AC is parallel to the ramp.) \ Now try Exercise 95. Section 6.3 Example 10 Recall from Section 4.8 that in air navigation, bearings can be measured in degrees clockwise from north. 453 Vectors in the Plane Using Vectors to Find Speed and Direction An airplane is traveling at a speed of 500 miles per hour with a bearing of 330! at a fixed altitude with a negligible wind velocity as shown in Figure 6.32(a). When the airplane reaches a certain point, it encounters a wind with a velocity of 70 miles per hour in the direction N 45! E, as shown in Figure 6.32(b). What are the resultant speed and direction of the airplane? y y v2 nd Wi v1 v1 v θ 120° x (a) x (b) FIGURE 6.32 Solution Using Figure 6.32, the velocity of the airplane (alone) is v1 500!cos 120!, sin 120!" ! 250, 2503 " and the velocity of the wind is v2 70!cos 45!, sin 45!" ! 352, 352 ". So, the velocity of the airplane (in the wind) is v v1 v2 ! 250 352, 2503 352 " & !200.5, 482.5" and the resultant speed of the airplane is v & 200.52 482.52 & 522.5 miles per hour. Finally, if tan is the direction angle of the flight path, you have & 482.5 200.5 & 2.4065 which implies that & 180! arctan2.4065 & 180! 67.4! 112.6!. So, the true direction of the airplane is approximately 270! 180! 112.6! 337.4!. Now try Exercise 101. 454 Chapter 6 Additional Topics in Trigonometry EXERCISES 6.3 See www.CalcChat.com for worked-out solutions to odd-numbered exercises. VOCABULARY: Fill in the blanks. 1. A ________ ________ ________ can be used to represent a quantity that involves both magnitude and direction. 2. The directed line segment PQ has ________ point P and ________ point Q. \ \ \ 3. The ________ of the directed line segment PQ is denoted by PQ . \ 4. The set of all directed line segments that are equivalent to a given directed line segment PQ is a ________ v in the plane. 5. In order to show that two vectors are equivalent, you must show that they have the same ________ and the same ________ . 6. The directed line segment whose initial point is the origin is said to be in ________ ________ . 7. A vector that has a magnitude of 1 is called a ________ ________ . 8. The two basic vector operations are scalar ________ and vector ________ . 9. The vector u v is called the ________ of vector addition. 10. The vector sum v1i v2 j is called a ________ ________ of the vectors i and j, and the scalars v1 and v2 are called the ________ and ________ components of v, respectively. SKILLS AND APPLICATIONS In Exercises 11 and 12, show that u and v are equivalent. y 11. 6 (0, 4) 4 (6, 5) u 4 y 12. (3, 3) u v (2, 4) 2 (0, 0) v −2 2 (4, 1) −4 −2 4 −4 6 (0, −5) (−3, −4) y 13. y 14. 4 1 (1, 3) 3 x −4 −3 −2 2 1 −2 (−4, −2) x −1 1 2 4 (3, 5) (−1, 4) x −2 (−1, −1) 2 4 −5 (−4, 4 5 (3, −2) −1) −2 −3 −4 −5 5, 1 5, 17 8, 9 9, 3 15, 12 9, 40 22. 1, 11 23. 1, 5 24. 3, 11 y v u (2, 2) v x x −3 −2 −1 x 4 v(3, −1) Terminal Point 19. 3, 5 20. 2, 7 21. 1, 3 5 3 2 1 v 2 −4 1 2 y 16. 6 v In Exercises 25–30, use the figure to sketch a graph of the specified vector. To print an enlarged copy of the graph, go to the website www.mathgraphs.com. −3 3 y 15. −1 v v (3, 3) Initial Point In Exercises 13–24, find the component form and the magnitude of the vector v. 4 3 2 1 x −2 −1 y 18. 4 3 2 1 −2 −3 −2 x 4 x 2 y 17. 1 2 3 25. v 27. u v 29. u v 26. 5v 28. u 2v 30. v 21u Section 6.3 In Exercises 31–38, find (a) u # v, (b) u $ v, and (c) 2u $ 3v, Then sketch each resultant vector. 31. u !2, 1", v !1, 3" 33. u !5, 3", v !0, 0" 35. u i j, v 2i 3j 36. u 2i j, v 3j 37. u 2i, v j 32. u !2, 3", v !4, 0" 34. u !0, 0", v !2, 1" 38. u 2j, v 3i 39. u !3, 0" 41. v !2, 2" 43. v i j 40. u !0, 2" 42. v !5, 12" 44. v 6i 2j 45. w 4j 47. w i 2j 46. w 6i 48. w 7j 3i In Exercises 49–52, find the vector v with the given magnitude and the same direction as u. Magnitude Direction 49. v 10 50. v 3 u !3, 4" u !12, 5" 51. v 9 52. v 8 u !2, 5" u !3, 3" In Exercises 53–56, the initial and terminal points of a vector are given. Write a linear combination of the standard unit vectors i and j. Initial Point 53. 2, 1 54. 0, 2 55. 6, 4 56. 1, 5 Terminal Point 3, 2 3, 6 0, 1 2, 3 In Exercises 57–62, find the component form of v and sketch the specified vector operations geometrically, where u " 2i $ j, and w " i # 2j. 3 2u 57. v 59. v u 2w 61. v 123u w 3 4w 58. v 60. v u w 62. v u 2w In Exercises 63–66, find the magnitude and direction angle of the vector v. 63. v 6i 6j 64. v 5i 4j 65. v 3cos 60!i sin 60!j 66. v 8cos 135!i sin 135!j 455 In Exercises 67–74, find the component form of v given its magnitude and the angle it makes with the positive x-axis. Sketch v. Magnitude 67. v 3 68. v 1 7 69. v 2 3 In Exercises 39– 48, find a unit vector in the direction of the given vector. Verify that the result has a magnitude of 1. Vectors in the Plane 70. v 4 71. v 23 72. v 43 Angle 0! 45! 150! 150! 45! 90! 73. v 3 v in the direction 3i 4j 74. v 2 v in the direction i 3j In Exercises 75–78, find the component form of the sum of u and v with direction angles u and v . Magnitude 75. u 5 v 5 76. u 4 v 4 77. u 20 v 50 78. u 50 v 30 Angle u 0! 90! u 60! v v 90! u 45! v 180! u 30! v 110! In Exercises 79 and 80, use the Law of Cosines to find the angle ! between the vectors. ( Assume 0! " ! " 180!.) 79. v i j, w 2i 2j 80. v i 2j, w 2i j RESULTANT FORCE In Exercises 81 and 82, find the angle between the forces given the magnitude of their resultant. (Hint: Write force 1 as a vector in the direction of the positive x-axis and force 2 as a vector at an angle with the positive x-axis.) Force 1 Force 2 Resultant Force 81. 45 pounds 60 pounds 90 pounds 82. 3000 pounds 1000 pounds 3750 pounds 83. VELOCITY A gun with a muzzle velocity of 1200 feet per second is fired at an angle of 6! above the horizontal. Find the vertical and horizontal components of the velocity. 84. Detroit Tigers pitcher Joel Zumaya was recorded throwing a pitch at a velocity of 104 miles per hour. If he threw the pitch at an angle of 35! below the horizontal, find the vertical and horizontal components of the velocity. (Source: Damon Lichtenwalner, Baseball Info Solutions) 456 Chapter 6 Additional Topics in Trigonometry 85. RESULTANT FORCE Forces with magnitudes of 125 newtons and 300 newtons act on a hook (see figure). The angle between the two forces is 45!. Find the direction and magnitude of the resultant of these forces. y 93. TOW LINE TENSION A loaded barge is being towed by two tugboats, and the magnitude of the resultant is 6000 pounds directed along the axis of the barge (see figure). Find the tension in the tow lines if they each make an 18! angle with the axis of the barge. 2000 newtons 125 newtons 45° 18° 30° − 45° 300 newtons x x 18° 900 newtons FIGURE FOR 85 FIGURE FOR 86 86. RESULTANT FORCE Forces with magnitudes of 2000 newtons and 900 newtons act on a machine part at angles of 30! and 45!, respectively, with the x-axis (see figure). Find the direction and magnitude of the resultant of these forces. 87. RESULTANT FORCE Three forces with magnitudes of 75 pounds, 100 pounds, and 125 pounds act on an object at angles of 30!, 45!, and 120!, respectively, with the positive x-axis. Find the direction and magnitude of the resultant of these forces. 94. ROPE TENSION To carry a 100-pound cylindrical weight, two people lift on the ends of short ropes that are tied to an eyelet on the top center of the cylinder. Each rope makes a 20! angle with the vertical. Draw a figure that gives a visual representation of the situation, and find the tension in the ropes. In Exercises 95–98, a force of F pounds is required to pull an object weighing W pounds up a ramp inclined at degrees from the horizontal. 88. RESULTANT FORCE Three forces with magnitudes of 70 pounds, 40 pounds, and 60 pounds act on an object at angles of 30!, 45!, and 135!, respectively, with the positive x-axis. Find the direction and magnitude of the resultant of these forces. 95. Find F if W 100 pounds and 96. Find W if F 600 pounds and 89. A traffic light weighing 12 pounds is suspended by two cables (see figure). Find the tension in each cable. 99. WORK A heavy object is pulled 30 feet across a floor, using a force of 100 pounds. The force is exerted at an angle of 50! above the horizontal (see figure). Find the work done. (Use the formula for work, W FD, where F is the component of the force in the direction of motion and D is the distance.) θ 1 = 20° θ 2 = 20° 97. Find if F 5000 pounds and W 15,000 pounds. 98. Find F if W 5000 pounds and 100 lb 1 40! and 2 35!. 30 ft CABLE TENSION In Exercises 91 and 92, use the figure to determine the tension in each cable supporting the load. 91. A 50° 30° C 2000 lb B 92. 10 in. 20 in. B A 24 in. C 5000 lb FIGURE FOR 26!. Tension u 45° 50° 90. Repeat Exercise 89 if 12!. 14!. 99 1 lb FIGURE FOR 100 100. ROPE TENSION A tetherball weighing 1 pound is pulled outward from the pole by a horizontal force u until the rope makes a 45! angle with the pole (see figure). Determine the resulting tension in the rope and the magnitude of u. Section 6.3 101. NAVIGATION An airplane is flying in the direction of 148!, with an airspeed of 875 kilometers per hour. Because of the wind, its groundspeed and direction are 800 kilometers per hour and 140!, respectively (see figure). Find the direction and speed of the wind. 111. PROOF Prove that cos i sin j is a unit vector for any value of . 112. CAPSTONE The initial and terminal points of vector v are 3, 4 and 9, 1, respectively. y (a) Write v in component form. N 140° 148° W (b) Write v as the linear combination of the standard unit vectors i and j. (c) Sketch v with its initial point at the origin. E S Win d x 457 Vectors in the Plane (d) Find the magnitude of v. 800 kilometers per hour 875 kilometers per hour 113. GRAPHICAL REASONING Consider two forces F1 !10, 0" and F2 5!cos , sin ". 102. NAVIGATION A commercial jet is flying from Miami to Seattle. The jet’s velocity with respect to the air is 580 miles per hour, and its bearing is 332!. The wind, at the altitude of the plane, is blowing from the southwest with a velocity of 60 miles per hour. (a) Draw a figure that gives a visual representation of the situation. (b) Write the velocity of the wind as a vector in component form. (c) Write the velocity of the jet relative to the air in component form. (d) What is the speed of the jet with respect to the ground? (e) What is the true direction of the jet? EXPLORATION (a) Find F1 F2 as a function of . (b) Use a graphing utility to graph the function in part (a) for 0 " < 2. (c) Use the graph in part (b) to determine the range of the function. What is its maximum, and for what value of does it occur? What is its minimum, and for what value of does it occur? (d) Explain why the magnitude of the resultant is never 0. 114. TECHNOLOGY Write a program for your graphing utility that graphs two vectors and their difference given the vectors in component form. In Exercises 115 and 116, use the program in Exercise 114 to find the difference of the vectors shown in the figure. y 115. TRUE OR FALSE? In Exercises 103–110, use the figure to determine whether the statement is true or false. Justify your answer. 8 6 b (1, 6) 125 (4, 5) 4 2 (−20, 70) (5, 2) x x 2 c u (80, 80) (10, 60) (9, 4) t a y 116. 4 6 (−100, 0) 8 50 −50 w d s v 103. a d 104. c s 105. a u c 106. v w s 107. a w 2d 109. u v 2b t 108. a d 0 110. t w b a 117. WRITING In your own words, state the difference between a scalar and a vector. Give examples of each. 118. WRITING Give geometric descriptions of the operations of addition of vectors and multiplication of a vector by a scalar. 119. WRITING Identify the quantity as a scalar or as a vector. Explain your reasoning. (a) The muzzle velocity of a bullet (b) The price of a company’s stock (c) The air temperature in a room (d) The weight of an automobile 458 Chapter 6 Additional Topics in Trigonometry 6.4 VECTORS AND DOT PRODUCTS What you should learn • Find the dot product of two vectors and use the properties of the dot product. • Find the angle between two vectors and determine whether two vectors are orthogonal. • Write a vector as the sum of two vector components. • Use vectors to find the work done by a force. The Dot Product of Two Vectors So far you have studied two vector operations—vector addition and multiplication by a scalar—each of which yields another vector. In this section, you will study a third vector operation, the dot product. This product yields a scalar, rather than a vector. Definition of the Dot Product The dot product of u !u1, u2" and v !v1, v2" is u # v u1v1 u2v2. Why you should learn it You can use the dot product of two vectors to solve real-life problems involving two vector quantities. For instance, in Exercise 76 on page 466, you can use the dot product to find the force necessary to keep a sport utility vehicle from rolling down a hill. Properties of the Dot Product Let u, v, and w be vectors in the plane or in space and let c be a scalar. 1. u # v v # u 2. 0 # v 0 3. u # v w u # v u # w 4. v # v v 2 5. cu # v cu # v u # cv For proofs of the properties of the dot product, see Proofs in Mathematics on page 490. Example 1 Finding Dot Products Joe Raedle/Getty Images Find each dot product. a. !4, 5" # !2, 3" b. !2, 1" # !1, 2" c. !0, 3" # !4, 2" Solution a. !4, 5" # !2, 3" 42 53 8 15 23 b. !2, 1" # !1, 2" 21 12 220 c. !0, 3" # !4, 2" 04 32 0 6 6 Now try Exercise 7. In Example 1, be sure you see that the dot product of two vectors is a scalar (a real number), not a vector. Moreover, notice that the dot product can be positive, zero, or negative. Section 6.4 Example 2 Vectors and Dot Products 459 Using Properties of Dot Products Let u !1, 3", v !2, 4", and w !1, 2". Find each dot product. a. u # vw b. u # 2v Solution Begin by finding the dot product of u and v. u # v !1, 3" # !2, 4" 12 34 14 a. u # vw 14!1, 2" !14, 28" b. u # 2v 2u # v 214 28 Notice that the product in part (a) is a vector, whereas the product in part (b) is a scalar. Can you see why? Now try Exercise 17. Example 3 Dot Product and Magnitude The dot product of u with itself is 5. What is the magnitude of u? Solution Because u 2 u # u and u # u 5, it follows that u u # u 5. Now try Exercise 25. The Angle Between Two Vectors The angle between two nonzero vectors is the angle , 0 " " , between their respective standard position vectors, as shown in Figure 6.33. This angle can be found using the dot product. v−u u θ v Origin FIGURE 6.33 Angle Between Two Vectors If is the angle between two nonzero vectors u and v, then cos u u #v . v For a proof of the angle between two vectors, see Proofs in Mathematics on page 490. 460 Chapter 6 Additional Topics in Trigonometry Finding the Angle Between Two Vectors Example 4 Find the angle ! between u #4, 3$ and v #3, 5$. Solution y The two vectors and ! are shown in Figure 6.34. 6 5 4 #4, 3$ #4, 3$ u = 〈4, 3〉 3 2 θ 1 #3, 5$ #3, 5$ 27 534 This implies that the angle between the two vectors is x 1 FIGURE v v u u cos ! v = 〈3, 5〉 2 3 4 5 6 ! arccos 6.34 27 " 22.2#. 534 Now try Exercise 35. Rewriting the expression for the angle between two vectors in the form u v u v cos ! Alternative form of dot product produces an alternative way to calculate the dot product. From this form, you can see that because u and v are always positive, u v and cos ! will always have the same sign. Figure 6.35 shows the five possible orientations of two vectors. u θ u ! cos ! 1 Opposite Direction FIGURE 6.35 v u θ u θ < ! < 2 1 < cos ! < 0 Obtuse Angle θ 2 cos ! 0 90# Angle ! v v v v 0 < ! < 2 0 < cos ! < 1 Acute Angle u !0 cos ! 1 Same Direction Definition of Orthogonal Vectors The vectors u and v are orthogonal if u v 0. The terms orthogonal and perpendicular mean essentially the same thing—meeting at right angles. Note that the zero vector is orthogonal to every vector u, because 0 u 0. Section 6.4 Example 5 T E C H N O LO G Y The graphing utility program, Finding the Angle Between Two Vectors, found on the website for this text at academic.cengage.com, graphs two vectors u #a, b$ and v #c, d$ in standard position and finds the measure of the angle between them. Use the program to verify the solutions for Examples 4 and 5. Vectors and Dot Products 461 Determining Orthogonal Vectors Are the vectors u #2, 3$ and v #6, 4$ orthogonal? Solution Find the dot product of the two vectors. u v #2, 3$ #6, 4$ 26 34 0 Because the dot product is 0, the two vectors are orthogonal (see Figure 6.36). y v = 〈6, 4〉 4 3 2 1 x 1 2 3 4 5 6 7 −1 −2 u = 〈2, −3〉 −3 FIGURE 6.36 Now try Exercise 53. Finding Vector Components You have already seen applications in which two vectors are added to produce a resultant vector. Many applications in physics and engineering pose the reverse problem—decomposing a given vector into the sum of two vector components. Consider a boat on an inclined ramp, as shown in Figure 6.37. The force F due to gravity pulls the boat down the ramp and against the ramp. These two orthogonal forces, w1 and w2, are vector components of F. That is, F w1 w2. Vector components of F The negative of component w1 represents the force needed to keep the boat from rolling down the ramp, whereas w2 represents the force that the tires must withstand against the ramp. A procedure for finding w1 and w2 is shown on the following page. w1 w2 F FIGURE 6.37 462 Chapter 6 Additional Topics in Trigonometry Definition of Vector Components Let u and v be nonzero vectors such that u w1 w2 where w1 and w2 are orthogonal and w1 is parallel to (or a scalar multiple of) v, as shown in Figure 6.38. The vectors w1 and w2 are called vector components of u. The vector w1 is the projection of u onto v and is denoted by w1 proj v u. The vector w2 is given by w2 u w1. w2 u u θ w2 θ v w1 v w1 ! is acute. FIGURE 6.38 ! is obtuse. From the definition of vector components, you can see that it is easy to find the component w2 once you have found the projection of u onto v. To find the projection, you can use the dot product, as follows. u u w1 w2 cv w2 w1 is a scalar multiple of v. v cv w2 Take dot product of each side with v. v cv v w2 c v 2 v 0 w2 and v are orthogonal. So, c u v v 2 and w1 projv u cv v u v 2 v. Projection of u onto v Let u and v be nonzero vectors. The projection of u onto v is proj v u u v v 2 v. Section 6.4 Example 6 1 v = 〈6, 2〉 x 1 2 3 4 5 The projection of u onto v is 6 −1 w1 projvu w2 −2 u v v 2 v 40#6, 2$ %5, 5& 8 6 2 as shown in Figure 6.39. The other component, w2, is −3 −4 w2 u w1 #3, 5$ u = 〈3, −5〉 −5 FIGURE Decomposing a Vector into Components Solution w1 −1 463 Find the projection of u #3, 5$ onto v #6, 2$. Then write u as the sum of two orthogonal vectors, one of which is projvu. y 2 Vectors and Dot Products %65, 52& %95, 275&. So, 6.39 u w1 w2 %65, 52& %95, 275& #3, 5$. Now try Exercise 59. Example 7 Finding a Force A 200-pound cart sits on a ramp inclined at 30#, as shown in Figure 6.40. What force is required to keep the cart from rolling down the ramp? Solution Because the force due to gravity is vertical and downward, you can represent the gravitational force by the vector v 30° FIGURE F 200j. w1 F 6.40 Force due to gravity To find the force required to keep the cart from rolling down the ramp, project F onto a unit vector v in the direction of the ramp, as follows. v cos 30#i sin 30#j 3 2 i 1 j 2 Unit vector along ramp Therefore, the projection of F onto v is w1 proj v F F F v v 2 v vv 200 2 v 1 23 i 21 j. 100 The magnitude of this force is 100, and so a force of 100 pounds is required to keep the cart from rolling down the ramp. Now try Exercise 75. 464 Chapter 6 Additional Topics in Trigonometry Work The work W done by a constant force F acting along the line of motion of an object is given by W magnitude of forcedistance F \ PQ as shown in Figure 6.41. If the constant force F is not directed along the line of motion, as shown in Figure 6.42, the work W done by the force is given by W proj PQ F \ cos ! F F \ PQ Projection form for work \ proj PQ F cos ! F PQ \ \ PQ . Alternative form of dot product F F θ proj PQ F Q P P Q Force acts at angle ! with the line of motion. FIGURE 6.42 Force acts along the line of motion. FIGURE 6.41 This notion of work is summarized in the following definition. Definition of Work The work W done by a constant force F as its point of application moves along the vector PQ is given by either of the following. \ \ 1. W projPQ F PQ Projection form \ 2. W F Example 8 \ PQ Dot product form Finding Work To close a sliding barn door, a person pulls on a rope with a constant force of 50 pounds at a constant angle of 60#, as shown in Figure 6.43. Find the work done in moving the barn door 12 feet to its closed position. Solution Using a projection, you can calculate the work as follows. 12 ft projPQ F P Q W proj PQ F \ cos 60# F 60° \ PQ Projection form for work \ PQ 1 5012 300 foot-pounds 2 F So, the work done is 300 foot-pounds. You can verify this result by finding the vectors F and PQ and calculating their dot product. \ 12 ft FIGURE 6.43 Now try Exercise 79. Section 6.4 6.4 EXERCISES Vectors and Dot Products 465 See www.CalcChat.com for worked-out solutions to odd-numbered exercises. VOCABULARY: Fill in the blanks. 1. The ________ ________ of two vectors yields a scalar, rather than a vector. 2. The dot product of u #u1, u2$ and v #v1, v2$ is u v ________ . 3. If ! is the angle between two nonzero vectors u and v, then cos ! ________ . 4. The vectors u and v are ________ if u v 0. 5. The projection of u onto v is given by projvu ________ . \ 6. The work W done by a constant force F as its point of application moves along the vector PQ is given by W ________ or W ________ . SKILLS AND APPLICATIONS In Exercises 7–14, find the dot product of u and v. 7. u #7, 1$ v #3, 2$ 8. u #6, 10$ v #2, 3$ 9. u #4, 1$ v #2, 3$ 11. u 4i 2j 10. u #2, 5$ v #1, 8$ 12. u 3i 4j vij 37. u 5i 5j v 6i 6j 3 i sin 3 j 3 3 v cos i sin j 4 4 40. u cos i sin j 4 4 v cos i sin j 2 2 39. u cos v 7i 2j 13. u 3i 2j v 2i 3j 38. u 2i 3j v 4i 3j 14. u i 2j v 2i j In Exercises 15–24, use the vectors u #3, 3$, v #"4, 2$, and w #3, "1$ to find the indicated quantity. State whether the result is a vector or a scalar. In Exercises 41– 44, graph the vectors and find the degree measure of the angle ! between the vectors. 15. u u 17. u vv 19. 3w vu 16. 3u v 18. v uw 20. u 2vw 41. u 3i 4j v 7i 5j 43. u 5i 5j 21. w 1 23. u v u 22. 2 u 24. v u w w v 8i 8j v In Exercises 25–30, use the dot product to find the magnitude of u. 25. u #8, 15$ 27. u 20i 25j 29. u 6j 42. u 6i 3j v 4i 4j 44. u 2i 3j 26. u #4, 6$ 28. u 12i 16j 30. u 21i v 8i 3j In Exercises 45– 48, use vectors to find the interior angles of the triangle with the given vertices. 45. 1, 2, 3, 4, 2, 5 47. 3, 0, 2, 2, 0, 6 46. 3, 4, 1, 7, 8, 2 48. 3, 5, 1, 9, 7, 9 In Exercises 49 –52, find u u and v. v, where ! is the angle between 2 3 In Exercises 31– 40, find the angle ! between the vectors. 49. u 4, v 10, ! 31. u #1, 0$ v #0, 2$ 33. u 3i 4j 32. u #3, 2$ v #4, 0$ 34. u 2i 3j 50. u 100, v 250, ! v 2j 35. u 2i j v 6i 4j v i 2j 36. u 6i 3j v 8i 4j 51. u 9, v 36, ! 3 4 52. u 4, v 12, ! 3 6 466 Chapter 6 Additional Topics in Trigonometry In Exercises 53–58, determine whether u and v are orthogonal, parallel, or neither. 53. u #12, 30$ 5 1 v # 2, 4$ 54. u #3, 15$ 55. u 413i j v 5i 6j v #1, 5$ 56. u i v 2i 2j 57. u 2i 2j 58. u #cos !, sin ! $ (a) Find the dot product u v and interpret the result in the context of the problem. (b) Identify the vector operation used to increase the prices by 5%. v #sin !, cos ! $ v i j In Exercises 59–62, find the projection of u onto v. Then write u as the sum of two orthogonal vectors, one of which is projv u. 59. u #2, 2$ 60. u #4, 2$ v #6, 1$ 62. u #3, 2$ v #4, 1$ (b) Identify the vector operation used to increase the prices by 2.5%. In Exercises 63–66, use the graph to determine mentally the projection of u onto v. (The coordinates of the terminal points of the vectors in standard position are given.) Use the formula for the projection of u onto v to verify your result. y 63. 6 5 4 3 2 1 6 (6, 4) x −4 y 65. u (−3, −2) 6 (6, 4) v 2 4 Weight = 30,000 lb v 2 u −2 −2 2 4 6 (a) Find the force required to keep the truck from rolling down the hill in terms of the slope d. (b) Use a graphing utility to complete the table. (6, 4) 4 x −2 −2 −4 d° 6 y 66. (−2, 3) v 2 v 1 2 3 4 5 6 −1 (6, 4) 4 x 75. BRAKING LOAD A truck with a gross weight of 30,000 pounds is parked on a slope of d # (see figure). Assume that the only force to overcome is the force of gravity. y 64. (3, 2) u 74. REVENUE The vector u #3140, 2750$ gives the numbers of hamburgers and hot dogs, respectively, sold at a fast-food stand in one month. The vector v #2.25, 1.75$ gives the prices (in dollars) of the food items. (a) Find the dot product u v and interpret the result in the context of the problem. v #1, 2$ 61. u #0, 3$ v #2, 15$ 73. REVENUE The vector u #4600, 5250$ gives the numbers of units of two models of cellular phones produced by a telecommunications company. The vector v #79.99, 99.99$ gives the prices (in dollars) of the two models of cellular phones, respectively. x 2 u 4 6 (2, −3) d 67. u #3, 5$ 69. u 21 i 23 j 68. u #8, 3$ 5 70. u 2 i 3j WORK In Exercises 71 and 72, find the work done in moving a particle from P to Q if the magnitude and direction of the force are given by v. 71. P0, 0, 72. P1, 3, Q4, 7, v #1, 4$ Q3, 5, v 2i 3j 1# 2# 3# 4# 6# 7# 8# 9# 10# 5# Force d In Exercises 67–70, find two vectors in opposite directions that are orthogonal to the vector u. (There are many correct answers.) 0# Force (c) Find the force perpendicular to the hill when d 5#. 76. BRAKING LOAD A sport utility vehicle with a gross weight of 5400 pounds is parked on a slope of 10#. Assume that the only force to overcome is the force of gravity. Find the force required to keep the vehicle from rolling down the hill. Find the force perpendicular to the hill. Section 6.4 Vectors and Dot Products 467 77. WORK Determine the work done by a person lifting a 245-newton bag of sugar 3 meters. 84. PROGRAMMING Use the program you wrote in Exercise 83 to find the angle between the given vectors. 78. WORK Determine the work done by a crane lifting a 2400-pound car 5 feet. (a) u #8, 4$ and v #2, 5$ (b) u #2, 6$ and v #4, 1$ 85. PROGRAMMING Given vectors u and v in component form, write a program for your graphing utility in which the output is the component form of the projection of u onto v. 86. PROGRAMMING Use the program you wrote in Exercise 85 to find the projection of u onto v for the given vectors. 79. WORK A force of 45 pounds exerted at an angle of 30# above the horizontal is required to slide a table across a floor (see figure). The table is dragged 20 feet. Determine the work done in sliding the table. 45 lb (a) u #5, 6$ and v #1, 3$ (b) u #3, 2$ and v #2, 1$ 30° EXPLORATION 20 ft 80. WORK A tractor pulls a log 800 meters, and the tension in the cable connecting the tractor and log is approximately 15,691 newtons. The direction of the force is 35# above the horizontal. Approximate the work done in pulling the log. 81. WORK One of the events in a local strongman contest is to pull a cement block 100 feet. One competitor pulls the block by exerting a force of 250 pounds on a rope attached to the block at an angle of 30# with the horizontal (see figure). Find the work done in pulling the block. TRUE OR FALSE? In Exercises 87 and 88, determine whether the statement is true or false. Justify your answer. 87. The work W done by a constant force F acting along the line of motion of an object is represented by a vector. 88. A sliding door moves along the line of vector PQ . If a force is applied to the door along a vector that is orthogonal to PQ , then no work is done. \ \ 89. PROOF Use vectors to prove that the diagonals of a rhombus are perpendicular. 90. CAPSTONE What is known about !, the angle between two nonzero vectors u and v, under each condition (see figure)? 30° θ u 100 ft v Not drawn to scale 82. WORK A toy wagon is pulled by exerting a force of 25 pounds on a handle that makes a 20# angle with the horizontal (see figure). Find the work done in pulling the wagon 50 feet. Origin (a) u v0 (b) u v > 0 (c) u v < 0 91. THINK ABOUT IT What can be said about the vectors u and v under each condition? (a) The projection of u onto v equals u. (b) The projection of u onto v equals 0. 92. PROOF Prove the following. 20° uv 2 u 2 v 2 2u v 93. PROOF Prove that if u is a unit vector and ! is the angle between u and i, then u cos ! i sin ! j. 83. PROGRAMMING Given vectors u and v in component form, write a program for your graphing utility in which the output is (a) u , (b) v , and (c) the angle between u and v. 94. PROOF Prove that if u is a unit vector and ! is the angle between u and j, then u cos 2 ! i sin2 ! j. 468 Chapter 6 Additional Topics in Trigonometry 6.5 TRIGONOMETRIC FORM OF A COMPLEX NUMBER What you should learn • Plot complex numbers in the complex plane and find absolute values of complex numbers. • Write the trigonometric forms of complex numbers. • Multiply and divide complex numbers written in trigonometric form. • Use DeMoivre’s Theorem to find powers of complex numbers. • Find nth roots of complex numbers. The Complex Plane Just as real numbers can be represented by points on the real number line, you can represent a complex number z a bi as the point a, b in a coordinate plane (the complex plane). The horizontal axis is called the real axis and the vertical axis is called the imaginary axis, as shown in Figure 6.44. Imaginary axis 3 Why you should learn it (3, 1) or 3+i 2 You can use the trigonometric form of a complex number to perform operations with complex numbers. For instance, in Exercises 99–106 on page 477, you can use the trigonometric forms of complex numbers to help you solve polynomial equations. 1 −3 −2 −1 −1 1 2 3 Real axis (−2, −1) or −2 −2 − i FIGURE 6.44 The absolute value of the complex number a bi is defined as the distance between the origin 0, 0 and the point a, b. Definition of the Absolute Value of a Complex Number The absolute value of the complex number z a bi is a bi a2 b2. If the complex number a bi is a real number (that is, if b 0), then this definition agrees with that given for the absolute value of a real number a 0i a2 02 a . Imaginary axis (−2, 5) Example 1 Plot z 2 5i and find its absolute value. 4 3 Solution 29 −4 −3 −2 −1 FIGURE 6.45 Finding the Absolute Value of a Complex Number 5 The number is plotted in Figure 6.45. It has an absolute value of 1 2 3 4 Real axis z 22 52 29. Now try Exercise 9. Section 6.5 Trigonometric Form of a Complex Number 469 Trigonometric Form of a Complex Number Imaginary axis In Section 2.4, you learned how to add, subtract, multiply, and divide complex numbers. To work effectively with powers and roots of complex numbers, it is helpful to write complex numbers in trigonometric form. In Figure 6.46, consider the nonzero complex number a bi. By letting ! be the angle from the positive real axis (measured counterclockwise) to the line segment connecting the origin and the point a, b, you can write (a , b) r b a r cos ! θ Real axis a where r a2 and b2. b r sin ! Consequently, you have a bi r cos ! r sin !i from which you can obtain the trigonometric form of a complex number. FIGURE 6.46 Trigonometric Form of a Complex Number The trigonometric form of the complex number z a bi is z rcos ! i sin ! where a r cos !, b r sin !, r a2 b2, and tan ! ba. The number r is the modulus of z, and ! is called an argument of z. The trigonometric form of a complex number is also called the polar form. Because there are infinitely many choices for !, the trigonometric form of a complex number is not unique. Normally, ! is restricted to the interval 0 % ! < 2, although on occasion it is convenient to use ! < 0. Example 2 Writing a Complex Number in Trigonometric Form Write the complex number z 2 23i in trigonometric form. Solution The absolute value of z is r 2 23 i 22 23 16 4 and the reference angle !$ is given by Imaginary axis −3 −2 4π 3 z = 4 1 FIGURE 6.47 Real axis tan !$ b 23 3. a 2 Because tan3 3 and because z 2 23i lies in Quadrant III, you choose ! to be ! 3 43. So, the trigonometric form is −2 −3 z = −2 − 2 3 i 2 −4 z r cos ! i sin ! 4 cos 4 4 . i sin 3 3 See Figure 6.47. Now try Exercise 17. 470 Chapter 6 Additional Topics in Trigonometry Example 3 Writing a Complex Number in Standard Form Write the complex number in standard form a bi. 3 i sin 3 z 8 cos Solution T E C H N O LO G Y A graphing utility can be used to convert a complex number in trigonometric (or polar) form to standard form. For specific keystrokes, see the user’s manual for your graphing utility. 3 21 and sin 3 23, you can write Because cos 3 i sin 3 z 8 cos 12 23i 22 2 6 i. Now try Exercise 35. Multiplication and Division of Complex Numbers The trigonometric form adapts nicely to multiplication and division of complex numbers. Suppose you are given two complex numbers z1 r1cos !1 i sin !1 and z 2 r2cos !2 i sin !2 . The product of z1 and z 2 is given by z1z2 r1r2cos !1 i sin !1cos !2 i sin !2 r1r2cos !1 cos !2 sin !1 sin !2 isin !1 cos !2 cos !1 sin !2 . Using the sum and difference formulas for cosine and sine, you can rewrite this equation as z1z2 r1r2cos!1 !2 i sin!1 !2 . This establishes the first part of the following rule. The second part is left for you to verify (see Exercise 109). Product and Quotient of Two Complex Numbers Let z1 r1cos !1 i sin !1 and z2 r2cos !2 i sin !2 be complex numbers. z1z2 r1r2cos!1 !2 i sin!1 !2 z1 r1 cos!1 !2 i sin!1 !2 , z2 r2 Product z2 0 Quotient Note that this rule says that to multiply two complex numbers you multiply moduli and add arguments, whereas to divide two complex numbers you divide moduli and subtract arguments. Section 6.5 Example 4 Trigonometric Form of a Complex Number 471 Multiplying Complex Numbers Find the product z1z2 of the complex numbers. z1 2 cos 2 2 i sin 3 3 z 2 8 cos 11 11 i sin 6 6 Solution T E C H N O LO G Y Some graphing utilities can multiply and divide complex numbers in trigonometric form. If you have access to such a graphing utility, use it to find z1z2 and z1z2 in Examples 4 and 5. z1z 2 2 cos 2 2 i sin 3 3 3 2 16 cos ! 8cos 5 5 i sin 2 2 i sin 2 2 16 cos 2 11 11 i sin 6 3 6 16 cos 11 11 i sin 6 6 Multiply moduli and add arguments. 5 and are coterminal. 2 2 160 i1 16i You can check this result by first converting the complex numbers to the standard forms z1 1 3i and z2 43 4i and then multiplying algebraically, as in Section 2.4. z1z2 1 3 i43 4i 43 4i 12i 43 16i Now try Exercise 47. Example 5 Dividing Complex Numbers Find the quotient z1z 2 of the complex numbers. z1 24cos 300 i sin 300 z 2 8cos 75 i sin 75 Solution z1 24cos 300 i sin 300 z2 8cos 75 i sin 75 24 cos300 75 i sin300 75 8 3cos 225 i sin 225 2 i 2 3 2 2 32 32 i 2 2 Now try Exercise 53. Divide moduli and subtract arguments. 472 Chapter 6 Additional Topics in Trigonometry Powers of Complex Numbers The trigonometric form of a complex number is used to raise a complex number to a power. To accomplish this, consider repeated use of the multiplication rule. z r cos " i sin " z2 r cos " i sin "r cos " i sin " r 2cos 2" i sin 2" z3 r 2cos 2" i sin 2"r cos " i sin " r 3cos 3" i sin 3" z4 r 4cos 4" i sin 4" z5 r 5cos 5" i sin 5" .. . This pattern leads to DeMoivre’s Theorem, which is named after the French mathematician Abraham DeMoivre (1667–1754). HISTORICAL NOTE DeMoivre’s Theorem If z r cos " i sin " is a complex number and n is a positive integer, then zn r cos " i sin "n The Granger Collection r n cos n" i sin n". Example 6 Finding Powers of a Complex Number Use DeMoivre’s Theorem to find 1 3i . 12 Abraham DeMoivre (1667–1754) is remembered for his work in probability theory and DeMoivre’s Theorem. His book The Doctrine of Chances (published in 1718) includes the theory of recurring series and the theory of partial fractions. Solution First convert the complex number to trigonometric form using r 12 3 2 2 and " arctan 3 1 So, the trigonometric form is z 1 3 i 2 cos 2 2 . i sin 3 3 Then, by DeMoivre’s Theorem, you have 1 3 i12 2cos 212 cos 2 2 i sin 3 3 12 122 122 i sin 3 3 4096cos 8 i sin 8 40961 0 4096. Now try Exercise 69. 2 . 3 Section 6.5 Trigonometric Form of a Complex Number 473 Roots of Complex Numbers Recall that a consequence of the Fundamental Theorem of Algebra is that a polynomial equation of degree n has n solutions in the complex number system. So, the equation x6 1 has six solutions, and in this particular case you can find the six solutions by factoring and using the Quadratic Formula. x 6 1 x 3 1x 3 1 x 1x 2 x 1x 1x 2 x 1 0 Consequently, the solutions are x ± 1, x 1 ± 3 i , 2 and x 1 ± 3 i . 2 Each of these numbers is a sixth root of 1. In general, an nth root of a complex number is defined as follows. Definition of an nth Root of a Complex Number The complex number u a bi is an nth root of the complex number z if z un a bin. To find a formula for an nth root of a complex number, let u be an nth root of z, where u scos # i sin # and z r cos " i sin ". By DeMoivre’s Theorem and the fact that un z, you have sn cos n# i sin n# r cos " i sin ". Taking the absolute value of each side of this equation, it follows that sn r. Substituting back into the previous equation and dividing by r, you get cos n# i sin n# cos " i sin ". So, it follows that cos n# cos " and sin n# sin ". Because both sine and cosine have a period of 2, these last two equations have solutions if and only if the angles differ by a multiple of 2. Consequently, there must exist an integer k such that n# " 2 k # " 2k . n By substituting this value of # into the trigonometric form of u, you get the result stated on the following page. 474 Chapter 6 Additional Topics in Trigonometry Finding nth Roots of a Complex Number For a positive integer n, the complex number z rcos " i sin " has exactly n distinct nth roots given by n r cos " 2 k " 2 k i sin n n where k 0, 1, 2, . . . , n 1. Imaginary axis When k exceeds n 1, the roots begin to repeat. For instance, if k n, the angle " 2 n " 2 n n n FIGURE 2π n 2π n r Real axis 6.48 is coterminal with "n, which is also obtained when k 0. The formula for the nth roots of a complex number z has a nice geometrical interpretation, as shown in Figure 6.48. Note that because the nth roots of z all have the n n r, they all lie on a circle of radius r with center at the origin. same magnitude Furthermore, because successive nth roots have arguments that differ by 2n, the n roots are equally spaced around the circle. You have already found the sixth roots of 1 by factoring and by using the Quadratic Formula. Example 7 shows how you can solve the same problem with the formula for nth roots. Example 7 Finding the nth Roots of a Real Number Find all sixth roots of 1. Solution First write 1 in the trigonometric form 1 1cos 0 i sin 0. Then, by the nth root formula, with n 6 and r 1, the roots have the form 6 1 cos k k 0 2k 0 2k cos i sin i sin . 6 6 3 3 So, for k 0, 1, 2, 3, 4, and 5, the sixth roots are as follows. (See Figure 6.49.) cos 0 i sin 0 1 Imaginary axis 1 − + 3i 2 2 −1 + 0i −1 − 1 + 3i 2 2 1 + 0i 1 1 3i − 2 2 FIGURE cos 6.49 1 3i − 2 2 cos Real axis 1 3 i sin i 3 3 2 2 2 2 1 3 i sin i 3 3 2 2 cos i sin 1 cos 4 1 3 4 i sin i 3 3 2 2 cos 5 5 1 3 i sin i 3 3 2 2 Now try Exercise 91. Increment by 2 2 n 6 3 Section 6.5 Trigonometric Form of a Complex Number 475 In Figure 6.49, notice that the roots obtained in Example 7 all have a magnitude of 1 and are equally spaced around the unit circle. Also notice that the complex roots occur in conjugate pairs, as discussed in Section 2.5. The n distinct nth roots of 1 are called the nth roots of unity. Example 8 Finding the nth Roots of a Complex Number Find the three cube roots of z 2 2i. Solution Because z lies in Quadrant II, the trigonometric form of z is z 2 2i 8 cos 135 i sin 135 . " arctan 22 135 By the formula for nth roots, the cube roots have the form 6 8 cos 135 360 k 135º 360 k . i sin 3 3 Finally, for k 0, 1, and 2, you obtain the roots 6 8 cos 135 360 0 135 360 0 i sin 2 cos 45 i sin 45 3 3 1i 6 8 cos Imaginary axis 1+i −2 1 6 8 cos 1 2 Real axis −1 FIGURE 1.3660 0.3660i −1.3660 + 0.3660i −2 135 360 1 135 360 1 2cos 165 i sin 165 i sin 3 3 0.3660 − 1.3660i 135 360 2 135 360 2 2 cos 285 i sin 285 i sin 3 3 0.3660 1.3660i. See Figure 6.50. Now try Exercise 97. 6.50 CLASSROOM DISCUSSION A Famous Mathematical Formula The famous formula Note in Example 8 that the absolute value of z is r 2 2i 22 22 8 and the angle " is given by b 2 tan " 1. a 2 e abi " e acos b ! i sin b is called Euler’s Formula, after the Swiss mathematician Leonhard Euler (1707–1783). Although the interpretation of this formula is beyond the scope of this text, we decided to include it because it gives rise to one of the most wonderful equations in mathematics. e i !1"0 This elegant equation relates the five most famous numbers in mathematics —0, 1, , e, and i—in a single equation. Show how Euler’s Formula can be used to derive this equation. 476 Chapter 6 Additional Topics in Trigonometry EXERCISES 6.5 See www.CalcChat.com for worked-out solutions to odd-numbered exercises. VOCABULARY: Fill in the blanks. 1. The ________ ________ of a complex number a bi is the distance between the origin 0, 0 and the point a, b. 2. The ________ ________ of a complex number z a bi is given by z r cos " i sin ", where r is the ________ of z and " is the ________ of z. 3. ________ Theorem states that if z r cos " i sin " is a complex number and n is a positive integer, then z n r ncos n" i sin n". 4. The complex number u a bi is an ________ ________ of the complex number z if z un a bin. SKILLS AND APPLICATIONS In Exercises 5–10, plot the complex number and find its absolute value. 5. 6 8i 7. 7i 9. 4 6i 33. 2cos 60 i sin 60 34. 5cos 135 i sin 135 35. 48 cos30 i sin30 6. 5 12i 8. 7 10. 8 3i 36. 8cos 225 i sin 225 In Exercises 11–14, write the complex number in trigonometric form. 11. 4 3 2 1 −2 −1 13. 12. Imaginary axis 4 1 2 −6 −4 −2 Real axis Imaginary axis Real axis −3 − 2 2 3 z = −1 + 3i −3 −2 −1 9 3 3 i sin cos 4 4 4 12 i sin 12 40. 8cos i sin 2 2 38. 6 cos 5 5 41. 5cos 198 45% i sin198 45% 42. 9.75cos280º 30% i sin280º 30% In Exercises 43–46, use a graphing utility to represent the complex number in standard form. Imaginary axis −2 −3 z = −3 − 3i Real axis −4 14. 37. 39. 7cos 0 i sin 0 Imaginary axis z = −2 2 z = 3i In Exercises 33–42, find the standard form of the complex number. Then represent the complex number graphically. Real axis 2 2 44. 10 cos i sin i sin 9 9 5 5 45. 2cos 155 i sin 155 46. 9cos 58º i sin 58º 43. 5 cos In Exercises 47–58, perform the operation and leave the result in trigonometric form. In Exercises 15–32, represent the complex number graphically, and find the trigonometric form of the number. 47. 2cos 4 i sin 4 6cos 12 i sin 12 15. 1 i 16. 5 5i 48. 17. 1 3 i 19. 21 3 i 18. 4 43 i 20. 523 i 4cos 3 i sin 3 4cos 21. 5i 22. 12i 23. 7 4i 25. 2 24. 3 i 26. 4 27. 22 i 28. 3 i 53. 29. 5 2i 31. 8 53i 30. 8 3i 32. 9 210 i 55. 49. 3 5 3 cos 1 2 cos 120 i sin 120 3 3 i sin 4 4 2 3 cos 30 i sin 30 4 5 cos 300 i sin 300 50. 100 i sin 100 51. cos 80 i sin 80 cos 330 i sin 330 52. cos 5 i sin 5 cos 20 i sin 20 3cos 50 i sin 50 9cos 20 i sin 20 54. cos 120 i sin 120 2cos 40 i sin 40 cos i sin 5cos 4.3 i sin 4.3 56. 4cos 2.1 i sin 2.1 cos3 i sin3 6cos 40 i sin 40 12cos 92 i sin 92 57. 58. 2cos 122 i sin 122 7cos 100 i sin 100 Section 6.5 In Exercises 59–64, (a) write the trigonometric forms of the complex numbers, (b) perform the indicated operation using the trigonometric forms, and (c) perform the indicated operation using the standard forms, and check your result with that of part (b). 60. 3 i1 i 62. 3i1 2 i 59. 2 2i1 i 61. 2i1 i 3 4i 63. 1 3 i 1 3 i 6 3i 64. 2 2 1 i 66. z 1 1 3i 2 68. 2 2i6 70. 3 2i8 3 72. 41 3 i 73. 5cos 20 i sin 20 3 74. 3cos 60 i sin 60 4 12 8 75. cos i sin 76. 2 cos i sin 4 4 2 2 77. 5cos 3.2 i sin 3.24 79. 3 2i5 78. cos 0 i sin 020 3 80. 5 4i 81. 3cos 15 i sin 15 4 82. 2cos 8 i sin 8 In Exercises 99–106, use the formula on page 474 to find all the solutions of the equation and represent the solutions graphically. 99. x 4 i 0 101. x 5 243 0 100. x3 1 0 102. x3 27 0 103. x 4 16i 0 105. x3 1 i 0 104. x 6 64i 0 106. x 4 1 i 0 TRUE OR FALSE? In Exercises 107 and 108, determine whether the statement is true or false. Justify your answer. 107. Geometrically, the nth roots of any complex number z are all equally spaced around the unit circle centered at the origin. In Exercises 67–82, use DeMoivre’s Theorem to find the indicated power of the complex number. Write the result in standard form. 67. 1 i5 69. 1 i6 10 71. 23 i 477 EXPLORATION In Exercises 65 and 66, represent the powers z, z2, z 3, and z 4 graphically. Describe the pattern. 65. z Trigonometric Form of a Complex Number 6 In Exercises 83–98, (a) use the formula on page 474 to find the indicated roots of the complex number, (b) represent each of the roots graphically, and (c) write each of the roots in standard form. 83. Square roots of 5cos 120 i sin 120 84. Square roots of 16cos 60 i sin 60 5 5 i sin 86. Fifth roots of 32cos 6 6 2 2 i sin 85. Cube roots of 8 cos 3 3 87. Cube roots of 125 2 1 3 i 88. Cube roots of 421 i 89. Square roots of 25i 91. Fourth roots of 16 90. Fourth roots of 625i 92. Fourth roots of i 93. Fifth roots of 1 95. Cube roots of 125 97. Fifth roots of 41 i 94. Cube roots of 1000 96. Fourth roots of 4 98. Sixth roots of 64i 108. The product of two complex numbers is zero only when the modulus of one (or both) of the complex numbers is zero. 109. Given two complex numbers z1 r1cos "1 i sin "1 and z2 r2cos "2 i sin "2, z2 0, show that z1 r 1 cos"1 "2 i sin"1 "2. z 2 r2 110. Show that z r cos " i sin " is the complex conjugate of z r cos " i sin ". 111. Use the trigonometric forms of z and z in Exercise 110 to find (a) zz and (b) zz, z 0. 112. Show that the negative of z rcos " i sin " is z r cos" i sin" . 113. Show that 121 3i is a ninth root of 1. 114. Show that 2141 i is a fourth root of 2. 115. THINK ABOUT IT Explain how you can use DeMoivre’s Theorem to solve the polynomial equation x 4 16 0. [Hint: Write 16 as 16cos i sin .] 116. CAPSTONE Use the graph of the roots of a complex number. (a) Write each of the roots in trigonometric form. (b) Identify the complex number whose roots are given. Use a graphing utility to verify your results. (i) (ii) Imaginary axis 30° 2 −1 2 2 Imaginary axis 3 30° 1 Real axis 3 45° 45° 45° 45° 3 3 Real axis 478 Chapter 6 Additional Topics in Trigonometry 6 CHAPTER SUMMARY What Did You Learn? Explanation/Examples Use the Law of Sines to solve oblique triangles (AAS or ASA) (p. 428). Law of Sines 1–12 If ABC is a triangle with sides a, b, and c, then a b c . sin A sin B sin C C Section 6.2 Section 6.1 b Section 6.3 Review Exercises C a h c A h a b B c A A is acute. B A is obtuse. 1–12 Use the Law of Sines to solve oblique triangles (SSA) (p. 430). If two sides and one opposite angle are given, three possible situations can occur: (1) no such triangle exists (see Example 4), (2) one such triangle exists (see Example 3), or (3) two distinct triangles may satisfy the conditions. (see Example 5). Find the areas of oblique triangles (p. 432). The area of any triangle is one-half the product of the lengths of two sides times the sine of their included angle. That is, Area 21 bc sin A 12 ab sin C 21 ac sin B. 13–16 Use the Law of Sines to model and solve real-life problems (p. 433). The Law of Sines can be used to approximate the total distance of a boat race course. (See Example 7.) 17–20 Use the Law of Cosines to solve oblique triangles (SSS or SAS) (p. 437). Law of Cosines 21–30 Standard Form Alternative Form a2 b2 c2 2bc cos A cos A b2 c2 a2 2bc b2 a2 c2 2ac cos B cos B a2 c2 b2 2ac c2 a2 b2 2ab cos C cos C a2 b2 c2 2ab Use the Law of Cosines to model and solve real-life problems (p. 439). The Law of Cosines can be used to find the distance between a pitcher’s mound and first base on a women’s softball field. (See Example 3.) 35–38 Use Heron’s Area Formula to find the area of a triangle (p. 440). Heron’s Area Formula: Given any triangle with sides of lengths a, b, and c, the area of the triangle is Area s s as bs c, where s a b c2. 39– 42 Represent vectors as directed line segments (p. 445). Initial point Write the component forms of vectors (p. 446). The component form of the vector with initial point Pp1, p2 and terminal point Qq1, q2 is given by \ P PQ Q Terminal point PQ !q1 p1, q2 p2" !v1, v2 " v. 43, 44 45–50 Section 6.4 Section 6.3 Chapter Summary What Did You Learn? Explanation/Examples Perform basic vector operations and represent them graphically (p. 447). Let u !u1, u2" and v !v1, v2" be vectors and let k be a scalar (a real number). 479 Review Exercises 51– 62 ku !ku1, ku2" u v !u1 v1, u2 v2" v !v1, v2" u v !u1 v1, u2 v2" Write vectors as linear combinations of unit vectors (p. 449). v !v1, v2 " v1!1, 0" v2!0, 1" v1i v2 j Find the direction angles of vectors (p. 451). If u 2i 2j, then the direction angle is tan " 22 1. So, " 45 . 69–74 Use vectors to model and solve real-life problems (p. 452). Vectors can be used to find the resultant speed and direction of an airplane. (See Example 10.) 75–78 Find the dot product of two vectors and use the properties of the dot product (p. 458). The dot product of u !u1, u2" and v !v1, v2" is 79–90 u ! v u1v1 u2v2. Find the angle between two vectors and determine whether two vectors are orthogonal (p. 459). If " is the angle between two nonzero vectors u and v, then u!v cos " . Vectors u and v are orthogonal if u ! v 0. #u# #v# 91–98 Write a vector as the sum of two vector components (p. 461). Many applications in physics and engineering require the decomposition of a given vector into the sum of two vector components. (See Example 7.) 99–102 Use vectors to find the work done by a force (p. 464). The work W done by a constant force F as its point of application moves along the vector PQ is given by either of the following. 1. W #projPQ F# #PQ # 2. W F ! PQ 103–106 Plot complex numbers in the complex plane and find absolute values of complex numbers (p. 468). A complex number z a bi can be represented as the point a, b in the complex plane. The horizontal axis is the real axis and the vertical axis is the imaginary axis. The absolute value of z a bi is a bi a2 b2. 107–112 Write the trigonometric forms of complex numbers (p. 469). The trigonometric form of the complex number z a bi is z r cos " i sin " where a r cos ", b r sin ", r a2 b2, and tan " ba. 113 –118 Multiply and divide complex numbers written in trigonometric form (p. 470). Let z1 r1cos "1 i sin "1 and z2 r2cos "2 i sin "2. 119, 120 Use DeMoivre’s Theorem to find powers of complex numbers (p. 472). DeMoivre’s Theorem: If z rcos " i sin " is a complex number and n is a positive integer, then Find nth roots of complex numbers (p. 473). The complex number u a bi is an nth root of the complex number z if z un a bin. 63– 68 The scalars v1 and v2 are the horizontal and vertical components of v, respectively. The vector sum v1i v2 j is the linear combination of the vectors i and j. \ \ \ Section 6.5 \ z1z2 r1r2cos"1 "2 i sin"1 "2 z1z2 r1r2cos"1 "2 i sin"1 "2, z2 0 121–124 zn r cos " i sin "n r ncos n" i sin n". 125–134 480 Chapter 6 Additional Topics in Trigonometry 6 REVIEW EXERCISES See www.CalcChat.com for worked-out solutions to odd-numbered exercises. 6.1 In Exercises 1–12, use the Law of Sines to solve (if possible) the triangle. If two solutions exist, find both. Round your answers to two decimal places. 1. c A 70° a = 8 38° b 75 B 2. B c a = 19 121° 22° C b A ft 45° C 28° 3. B 72 , C 82 , b 54 FIGURE FOR 4. B 10 , C 20 , c 33 5. A 16 , B 98 , c 8.4 20. RIVER WIDTH A surveyor finds that a tree on the opposite bank of a river flowing due east has a bearing of N 22 30% E from a certain point and a bearing of N 15 W from a point 400 feet downstream. Find the width of the river. 6. A 95 , B 45 , c 104.8 7. A 24 , C 48 , b 27.5 8. B 64 , C 36 , a 367 9. B 150 , b 30, c 10 6.2 In Exercises 21–30, use the Law of Cosines to solve the triangle. Round your answers to two decimal places. 10. B 150 , a 10, b 3 11. A 75 , a 51.2, b 33.7 21. 12. B 25 , a 6.2, b 4 C b = 14 In Exercises 13–16, find the area of the triangle having the indicated angle and sides. A 22. 13. A 33 , b 7, c 10 14. B 80 , a 4, c 8 15. C 119 , a 18, b 6 16. A 11 , b 22, c 21 17. HEIGHT From a certain distance, the angle of elevation to the top of a building is 17 . At a point 50 meters closer to the building, the angle of elevation is 31 . Approximate the height of the building. 18. GEOMETRY Find the length of the side w of the parallelogram. 12 w 19 c = 17 a=8 B C b = 4 100° a = 7 B A c 23. a 6, b 9, c 14 24. a 75, b 50, c 110 25. a 2.5, b 5.0, c 4.5 26. a 16.4, b 8.8, c 12.2 27. B 108 , a 11, c 11 28. B 150 , a 10, c 20 29. C 43 , a 22.5, b 31.4 30. A 62 , b 11.34, c 19.52 140° 16 19. HEIGHT A tree stands on a hillside of slope 28 from the horizontal. From a point 75 feet down the hill, the angle of elevation to the top of the tree is 45 (see figure). Find the height of the tree. In Exercises 31–34, determine whether the Law of Sines or the Law of Cosines is needed to solve the triangle. Then solve the triangle. 31. b 9, c 13, C 64 32. a 4, c 5, B 52 33. a 13, b 15, c 24 34. A 44 , B 31 , c 2.8 481 Review Exercises 35. GEOMETRY The lengths of the diagonals of a parallelogram are 10 feet and 16 feet. Find the lengths of the sides of the parallelogram if the diagonals intersect at an angle of 28 . 36. GEOMETRY The lengths of the diagonals of a parallelogram are 30 meters and 40 meters. Find the lengths of the sides of the parallelogram if the diagonals intersect at an angle of 34 . 37. SURVEYING To approximate the length of a marsh, a surveyor walks 425 meters from point A to point B. Then the surveyor turns 65 and walks 300 meters to point C (see figure). Approximate the length AC of the marsh. B In Exercises 45–50, find the component form of the vector v satisfying the conditions. y 45. y 46. 6 6 4 4 2 2 (6, 27 ) (−5, 4) v (0, 1) x −4 v 2 −2 (2, −1) x 4 6 −2 47. Initial point: 0, 10; terminal point: 7, 3 48. Initial point: 1, 5; terminal point: 15, 9 49. #v # 8, " 120 1 50. #v # 2, " 225 65° In Exercises 51–58, find (a) u 1 v, (b) u $ v, (c) 4 u, and (d) 3v ! 5u. 425 m 300 m 51. u !1, 3", v !3, 6" 52. u !4, 5", v !0, 1" A C 38. NAVIGATION Two planes leave an airport at approximately the same time. One is flying 425 miles per hour at a bearing of 355 , and the other is flying 530 miles per hour at a bearing of 67 . Draw a figure that gives a visual representation of the situation and determine the distance between the planes after they have flown for 2 hours. In Exercises 39–42, use Heron’s Area Formula to find the area of the triangle. 6.3 In Exercises 43 and 44, show that u and v are equivalent. 6 4 (− 2, 1) y 44. (4, 6) 4 (− 3, 2) 2 u u (6, 3) v (0, − 2) 6 (1, 4) v x −4 60. w 4u 5v 1 62. w 2 v 2 4 63. u !1, 5" 64. u !6, 8" 65. u has initial point 3, 4 and terminal point 9, 8. 66. u has initial point 2, 7 and terminal point 5, 9. In Exercises 67 and 68, write the vector v in the form #v #$cos # i ! sin # j%. 67. v 10i 10j 68. v 4i j (3, − 2) x −2 −2 In Exercises 59–62, find the component form of w and sketch the specified vector operations geometrically, where u " 6i $ 5j and v " 10i ! 3j. In Exercises 63–66, write vector u as a linear combination of the standard unit vectors i and j. 3 5 4 42. a 5, b 4, c 8 y 56. u 7i 3j, v 4i j 57. u 4i, v i 6j 58. u 6j, v i j 59. w 2u v 61. w 3v 39. a 3, b 6, c 8 40. a 15, b 8, c 10 41. a 12.3, b 15.8, c 3.7 43. 53. u !5, 2", v !4, 4" 54. u !1, 8", v !3, 2" 55. u 2i j, v 5i 3j (−1, − 4) In Exercises 69–74, find the magnitude and the direction angle of the vector v. 69. v 7cos 60 i sin 60 j 70. v 3cos 150 i sin 150 j 71. v 5i 4j 72. v 4i 7j 482 Chapter 6 Additional Topics in Trigonometry 73. v 3i 3j 74. v 8i j 75. RESULTANT FORCE Forces with magnitudes of 85 pounds and 50 pounds act on a single point. The angle between the forces is 15 . Describe the resultant force. 76. ROPE TENSION A 180-pound weight is supported by two ropes, as shown in the figure. Find the tension in each rope. 30° 30° 180 lb 77. NAVIGATION An airplane has an airspeed of 430 miles per hour at a bearing of 135 . The wind velocity is 35 miles per hour in the direction of N 30 E. Find the resultant speed and direction of the airplane. 78. NAVIGATION An airplane has an airspeed of 724 kilometers per hour at a bearing of 30 . The wind velocity is 32 kilometers per hour from the west. Find the resultant speed and direction of the airplane. 6.4 In Exercises 79– 82, find the dot product of u and v. 79. u !6, 7" 80. u !7, 12" v !3, 9" 81. u 3i 7j v 11i 5j v !4, 14" 82. u 7i 2j v 16i 12j In Exercises 83–90, use the vectors u " !$4, 2" and v " !5, 1" to find the indicated quantity. State whether the result is a vector or a scalar. 83. 2u ! u 85. 4 #u# 84. 3u ! v 86. #v#2 87. uu ! v 88. u ! vv 89. u ! u u ! v 90. v ! v v ! u In Exercises 91–94, find the angle # between the vectors. 7 7 i sin j 4 4 5 5 v cos i sin j 6 6 91. u cos 92. u cos 45 i sin 45 j v cos 300 i sin 300 j 93. u ! 22, 4", v ! 2, 1" 94. u ! 3, 3 ", v ! 4, 33 " In Exercises 95 –98, determine whether u and v are orthogonal, parallel, or neither. 95. u !3, 8" v !8, 3" 96. u ! 4, 2" v !2, 4" 97. u i v i 2j 98. u 2i j v 3i 6j 1 1 In Exercises 99–102, find the projection of u onto v. Then write u as the sum of two orthogonal vectors, one of which is projv u. 99. u 100. u 101. u 102. u !4, 3", v !8, 2" !5, 6", v !10, 0" !2, 7", v !1, 1" !3, 5", v !5, 2" WORK In Exercises 103 and 104, find the work done in moving a particle from P to Q if the magnitude and direction of the force are given by v. 103. P5, 3, Q8, 9, v !2, 7" 104. P2, 9, Q12, 8, v 3i 6j 105. WORK Determine the work done (in foot-pounds) by a crane lifting an 18,000-pound truck 48 inches. 106. WORK A mover exerts a horizontal force of 25 pounds on a crate as it is pushed up a ramp that is 12 feet long and inclined at an angle of 20 above the horizontal. Find the work done in pushing the crate. 6.5 In Exercises 107–112, plot the complex number and find its absolute value. 107. 7i 109. 5 3i 108. 6i 110. 10 4i 111. 2 2 i 112. 2 2i In Exercises 113–118, write the complex number in trigonometric form. 113. 4i 115. 5 5i 117. 5 12i 114. 7 116. 5 12i 118. 33 3i In Exercises 119 and 120, (a) write the two complex numbers in trigonometric form, and (b) use the trigonometric forms to find z1z2 and z1/ z2, where z2 % 0. 119. z1 23 2i, z2 10i 120. z1 31 i, z2 23 i 483 Review Exercises In Exercises 121–124, use DeMoivre’s Theorem to find the indicated power of the complex number. Write the result in standard form. 144. The vectors u and v have the same magnitudes in the two figures. In which figure will the magnitude of the sum be greater? Give a reason for your answer. 5 cos y (b) v v u u x x In Exercises 125–128, (a) use the formula on page 474 to find the indicated roots of the complex number, (b) represent each of the roots graphically, and (c) write each of the roots in standard form. 125. Sixth roots of 729i 126. Fourth roots of 256i 127. Cube roots of 8 128. Fifth roots of 1024 In Exercises 129–134, use the formula on page 474 to find all solutions of the equation and represent the solutions graphically. 129. x 4 81 0 131. x 3 8i 0 y (a) 4 i sin 12 12 4 4 5 122. 2 cos i sin 15 15 123. 2 3i 6 124. 1 i 8 121. 130. x 5 32 0 132. x 4 64i 0 145. Give a geometric description of the scalar multiple ku of the vector u, for k > 0 and for k < 0. 146. Give a geometric description of the sum of the vectors u and v. GRAPHICAL REASONING In Exercises 147 and 148, use the graph of the roots of a complex number. (a) Write each of the roots in trigonometric form. (b) Identify the complex number whose roots are given. Use a graphing utility to verify your results. 147. 148. Imaginary axis Imaginary axis 133. x 5 x 3 x 2 1 0 134. x 5 4x 3 8x 2 32 0 2 4 −2 EXPLORATION TRUE OR FALSE? In Exercises 135–139, determine whether the statement is true or false. Justify your answer. 135. The Law of Sines is true if one of the angles in the triangle is a right angle. 136. When the Law of Sines is used, the solution is always unique. 137. If u is a unit vector in the direction of v, then v v u. 138. If v a i bj 0, then a b. 4 60° 60° −2 4 3 4 Real axis 30° 4 60° 3 60° 30°4 4 Real axis 149. The figure shows z1 and z2. Describe z1z2 and z1z2. Imaginary axis z2 z1 1 θ −1 θ 1 Real axis 139. x 3 i is a solution of the equation x2 8i 0. 150. One of the fourth roots of a complex number z is shown in the figure. 140. State the Law of Sines from memory. 141. State the Law of Cosines from memory. (a) How many roots are not shown? 142. What characterizes a vector in the plane? (b) Describe the other roots. 143. Which vectors in the figure appear to be equivalent? Imaginary axis y A x D z 30° 1 B C E −1 1 Real axis 484 Chapter 6 Additional Topics in Trigonometry 6 CHAPTER TEST See www.CalcChat.com for worked-out solutions to odd-numbered exercises. Take this test as you would take a test in class. When you are finished, check your work against the answers given in the back of the book. In Exercises 1–6, use the information to solve (if possible) the triangle. If two solutions exist, find both solutions. Round your answers to two decimal places. 1. A 24 , B 68 , a 12.2 3. A 24 , a 11.2, b 13.4 5. B 100 , a 15, b 23 240 mi 37° B 370 mi C 7. A triangular parcel of land has borders of lengths 60 meters, 70 meters, and 82 meters. Find the area of the parcel of land. 8. An airplane flies 370 miles from point A to point B with a bearing of 24 . It then flies 240 miles from point B to point C with a bearing of 37 (see figure). Find the distance and bearing from point A to point C. In Exercises 9 and 10, find the component form of the vector v satisfying the given conditions. 9. Initial point of v: 3, 7; terminal point of v: 11, 16 10. Magnitude of v: v 12; direction of v: u !3, 5" In Exercises 11–14, u its graph. 11. u v 24° A FIGURE FOR 8 2. B 110 , C 28 , a 15.6 4. a 4.0, b 7.3, c 12.4 6. C 121 , a 34, b 55 < > 2, 7 and v 12. u v < > !6, 5 . Find the resultant vector and sketch 13. 5u 3v 14. 4u 2v 15. Find a unit vector in the direction of u !24, 7". 16. Forces with magnitudes of 250 pounds and 130 pounds act on an object at angles of 45 and 60 , respectively, with the x-axis. Find the direction and magnitude of the resultant of these forces. 17. Find the angle between the vectors u !1, 5" and v !3, 2". 18. Are the vectors u !6, 10" and v !5, 3" orthogonal? 19. Find the projection of u !6, 7" onto v !5, 1". Then write u as the sum of two orthogonal vectors. 20. A 500-pound motorcycle is headed up a hill inclined at 12 . What force is required to keep the motorcycle from rolling down the hill when stopped at a red light? 21. Write the complex number z 5 5i in trigonometric form. 22. Write the complex number z 6cos 120 i sin 120 in standard form. In Exercises 23 and 24, use DeMoivre’s Theorem to find the indicated power of the complex number. Write the result in standard form. 23. 3cos 7 7 i sin 6 6 8 24. 3 3i6 25. Find the fourth roots of 2561 3i. 26. Find all solutions of the equation x 3 27i 0 and represent the solutions graphically. Cumulative Test for Chapters 4–6 6 CUMULATIVE TEST FOR CHAPTERS 4– 6 485 See www.CalcChat.com for worked-out solutions to odd-numbered exercises. Take this test as you would take a test in class. When you are finished, check your work against the answers given in the back of the book. 1. Consider the angle ! 120 . (a) Sketch the angle in standard position. (b) Determine a coterminal angle in the interval 0 , 360 . (c) Convert the angle to radian measure. (d) Find the reference angle !&. (e) Find the exact values of the six trigonometric functions of !. 2. Convert the angle ! 1.45 radians to degrees. Round the answer to one decimal place. 21 3. Find cos ! if tan ! 20 and sin ! < 0. y 4 In Exercises 4–6, sketch the graph of the function. (Include two full periods.) x 1 −3 −4 FIGURE FOR 7 3 4. f x 3 2 sin x 5. gx 1 tan x 2 2 6. hx secx 7. Find a, b, and c such that the graph of the function hx a cosbx c matches the graph in the figure. 1 8. Sketch the graph of the function f x 2 x sin x over the interval 3 % x % 3. In Exercises 9 and 10, find the exact value of the expression without using a calculator. 10. tanarcsin 5 9. tanarctan 4.9 3 11. Write an algebraic expression equivalent to sinarccos 2x. 12. Use the fundamental identities to simplify: cos 13. Subtract and simplify: 2 x csc x. cos ! sin ! 1 . cos ! sin ! 1 In Exercises 14–16, verify the identity. 14. cot 2 $sec2 $ 1 1 15. sinx y sinx y sin2 x sin2 y 1 16. sin2 x cos2 x 81 cos 4x In Exercises 17 and 18, find all solutions of the equation in the interval [0, 2"#. 17. 2 cos2 " cos " 0 18. 3 tan ! cot ! 0 19. Use the Quadratic Formula to solve the equation in the interval 0, 2: sin2 x 2 sin x 1 0. 3 12 20. Given that sin u 13, cos v 5, and angles u and v are both in Quadrant I, find tanu v. 21. If tan ! 2, find the exact value of tan2!. 1 4 ! 22. If tan ! , find the exact value of sin . 3 2 486 Chapter 6 Additional Topics in Trigonometry 23. Write the product 5 sin 3 4 7 ' cos 4 as a sum or difference. 24. Write cos 9x cos 7x as a product. In Exercises 25–28, use the information to solve the triangle shown in the figure. Round your answers to two decimal places. C a b A FIGURE FOR c 25. A 30 , a 9, b 8 27. A 30 , C 90 , b 10 B 25–28 26. A 30 , b 8, c 10 28. a 4.7, b 8.1, c 10.3 In Exercises 29 and 30, determine whether the Law of Sines or the Law of Cosines is needed to solve the triangle. Then solve the triangle. 29. A 45 , B 26 , c 20 30. a 1.2, b 10, C 80 31. Two sides of a triangle have lengths 7 inches and 12 inches. Their included angle measures 99 . Find the area of the triangle. 32. Find the area of a triangle with sides of lengths 30 meters, 41 meters, and 45 meters. 33. Write the vector u !7, 8" as a linear combination of the standard unit vectors i and j. 34. Find a unit vector in the direction of v i j. 35. Find u ' v for u 3i 4j and v i 2j. 36. Find the projection of u !8, 2" onto v !1, 5". Then write u as the sum of two orthogonal vectors. 37. Write the complex number 2 2i in trigonometric form. 38. Find the product of 4cos 30 i sin 30 6cos 120 i sin 120 . Write the answer in standard form. 39. Find the three cube roots of 1. 40. Find all the solutions of the equation x5 243 0. 41. A ceiling fan with 21-inch blades makes 63 revolutions per minute. Find the angular speed of the fan in radians per minute. Find the linear speed of the tips of the blades in inches per minute. 5 feet 12 feet FIGURE FOR 44 42. Find the area of the sector of a circle with a radius of 12 yards and a central angle of 105 . 43. From a point 200 feet from a flagpole, the angles of elevation to the bottom and top of the flag are 16 45& and 18 , respectively. Approximate the height of the flag to the nearest foot. 44. To determine the angle of elevation of a star in the sky, you get the star in your line of vision with the backboard of a basketball hoop that is 5 feet higher than your eyes (see figure). Your horizontal distance from the backboard is 12 feet. What is the angle of elevation of the star? 45. Write a model for a particle in simple harmonic motion with a displacement of 4 inches and a period of 8 seconds. 46. An airplane’s velocity with respect to the air is 500 kilometers per hour, with a bearing of 30 . The wind at the altitude of the plane has a velocity of 50 kilometers per hour with a bearing of N 60 E. What is the true direction of the plane, and what is its speed relative to the ground? 47. A force of 85 pounds exerted at an angle of 60 above the horizontal is required to slide an object across a floor. The object is dragged 10 feet. Determine the work done in sliding the object. PROOFS IN MATHEMATICS Law of Tangents Besides the Law of Sines and the Law of Cosines, there is also a Law of Tangents, which was developed by Francois Vie`te (1540–1603). The Law of Tangents follows from the Law of Sines and the sum-to-product formulas for sine and is defined as follows. Law of Sines a b c . sin A sin B sin C C b A c A B A is acute. c A B A is obtuse. Proof Let h be the altitude of either triangle found in the figure above. Then you have sin A h b or h b sin A sin B h a or h a sin B. Equating these two values of h, you have or a b . sin A sin B Note that sin A 0 and sin B 0 because no angle of a triangle can have a measure of 0 or 180 . In a similar manner, construct an altitude from vertex B to side AC (extended in the obtuse triangle), as shown at the left. Then you have a b a a a sin B b sin A C C b a b tan A B2 a b tan A B2 The Law of Tangents can be used to solve a triangle when two sides and the included angle are given (SAS). Before calculators were invented, the Law of Tangents was used to solve the SAS case instead of the Law of Cosines, because computation with a table of tangent values was easier. (p. 428) If ABC is a triangle with sides a, b, and c, then c B A is acute. C sin A h c or h c sin A sin C h a or h a sin C. Equating these two values of h, you have a a sin C c sin A b A c B or c a . sin A sin C By the Transitive Property of Equality you know that a b c . sin A sin B sin C A is obtuse. So, the Law of Sines is established. 487 Law of Cosines (p. 437) Standard Form Alternative Form a2 b2 c2 2bc cos A cos A b2 c2 a2 2bc b2 a2 c2 2ac cos B cos B a2 c2 b2 2ac c2 a2 b2 2ab cos C cos C a2 b2 c2 2ab Proof y To prove the first formula, consider the top triangle at the left, which has three acute angles. Note that vertex B has coordinates c, 0. Furthermore, C has coordinates x, y, where x b cos A and y b sin A. Because a is the distance from vertex C to vertex B, it follows that C = (x, y) b y a x c2 y 02 a a2 Distance Formula x c y 0 2 2 Square each side. a b cos A c b sin A 2 x x c A B = (c, 0) a2 a2 2 b2 b2 cos2 sin2 2 A 2bc cos A A cos2 A c2 c2 Substitute for x and y. b2 sin2 A 2bc cos A a2 b2 c2 2bc cos A. y y sin2 A cos2 A 1 b x c2 y 02 b Distance Formula b2 x c2 y 02 B Square each side. b2 a cos B c2 a sin B2 x c x A = (c, 0) 2 2 b2 a2 2 2 Substitute for x and y. 2 2 b a cos B 2ac cos B c a sin B sin2 B cos2 B c2 2ac cos B b2 a2 c2 2ac cos B. A similar argument is used to establish the third formula. 488 Factor out b2. To prove the second formula, consider the bottom triangle at the left, which also has three acute angles. Note that vertex A has coordinates c, 0. Furthermore, C has coordinates x, y, where x a cos B and y a sin B. Because b is the distance from vertex C to vertex A, it follows that C = (x, y) a Expand. Expand. Factor out a2. sin2 B cos2 B 1 Heron’s Area Formula (p. 440) Given any triangle with sides of lengths a, b, and c, the area of the triangle is Area ss as bs c where s abc . 2 Proof From Section 6.1, you know that Area 1 bc sin A 2 Formula for the area of an oblique triangle 1 Area2 b2c2 sin2 A 4 Square each side. 14 b c sin A 1 b c 1 cos A 4 1 1 bc1 cos A bc1 cos A. 2 2 Area 2 2 2 2 2 2 Take the square root of each side. Pythagorean Identity Factor. Using the Law of Cosines, you can show that abc 1 bc1 cos A 2 2 ' a b c 2 abc 1 bc1 cos A 2 2 ' abc . 2 and Letting s a b c2, these two equations can be rewritten as 1 bc1 cos A ss a 2 and 1 bc1 cos A s bs c. 2 By substituting into the last formula for area, you can conclude that Area ss as bs c. 489 Properties of the Dot Product (p. 458) Let u, v, and w be vectors in the plane or in space and let c be a scalar. 1. u ' v v ' u 2. 0 ' v 0 3. u ' v w u ' v u ' w 4. v ' v v 2 5. cu ' v cu ' v u ' cv Proof Let u !u1, u2", v !v1, v2", w !w1, w2 ", 0 !0, 0", and let c be a scalar. 1. u ' v u1v1 u2v2 v1u1 v2u2 v ' u ' v 0 ' v1 0 ' v2 0 3. u ' v w u ' !v1 w1, v2 w2" 2. 0 u1v1 w1 u2v2 w2 u1v1 u1w1 u2v2 u2w2 u1v1 u2v2 u1w1 u2w2 u ' v u ' w 4. v ' v v12 v22 v12 v22 v 2 5. cu ' v c!u1, u2" 2 ' !v1, v2" cu1v1 u2v2 cu1v1 cu2v2 !cu1, cu2" ' !v1, v2" cu ' v Angle Between Two Vectors (p. 459) If ! is the angle between two nonzero vectors u and v, then cos ! u'v . u v Proof Consider the triangle determined by vectors u, v, and v u, as shown in the figure. By the Law of Cosines, you can write v−u u θ v u 2 u 2 v 2 2 u v cos ! v v u ' v u u 2 v 2 2 u v cos ! Origin v u ' v v u ' u u 2 v 2 2 u v cos ! v ' v u ' v v ' u u ' u u 2 v 2 2 u v cos ! v 2 2u ' v u 2 u 2 v 2 2 u v cos ! u'v cos ! . u v 490 PROBLEM SOLVING This collection of thought-provoking and challenging exercises further explores and expands upon concepts learned in this chapter. 1. In the figure, a beam of light is directed at the blue mirror, reflected to the red mirror, and then reflected back to the blue mirror. Find the distance PT that the light travels from the red mirror back to the blue mirror. Red P ror mir T u u v v (v) α Q 6 ft Blue mirror 2. A triathlete sets a course to swim S 25 E from a point on shore to a buoy 34 mile away. After swimming 300 yards through a strong current, the triathlete is off course at a bearing of S 35 E. Find the bearing and distance the triathlete needs to swim to correct her course. (vi) uv uv (b) u !0, 1" v !3, 3" 1 α O (iv) (iii) u v (ii) v (c) u ! 1, 2" θ 25° (i) u (a) u !1, 1" v !1, 2" θ t f 4.7 5. For each pair of vectors, find the following. (d) u !2, 4" v !2, 3" v !5, 5" 6. A skydiver is falling at a constant downward velocity of 120 miles per hour. In the figure, vector u represents the skydiver’s velocity. A steady breeze pushes the skydiver to the east at 40 miles per hour. Vector v represents the wind velocity. Up 140 120 300 yd 100 35° 3 mi 4 25° Buoy N W 80 u E 60 S 40 3. A hiking party is lost in a national park. Two ranger stations have received an emergency SOS signal from the party. Station B is 75 miles due east of station A. The bearing from station A to the signal is S 60 E and the bearing from station B to the signal is S 75 W. (a) Draw a diagram that gives a visual representation of the problem. (b) Find the distance from each station to the SOS signal. (c) A rescue party is in the park 20 miles from station A at a bearing of S 80 E. Find the distance and the bearing the rescue party must travel to reach the lost hiking party. 4. You are seeding a triangular courtyard. One side of the courtyard is 52 feet long and another side is 46 feet long. The angle opposite the 52-foot side is 65 . (a) Draw a diagram that gives a visual representation of the situation. (b) How long is the third side of the courtyard? (c) One bag of grass seed covers an area of 50 square feet. How many bags of grass seed will you need to cover the courtyard? v 20 W E −20 20 40 60 Down (a) Write the vectors u and v in component form. (b) Let s u v. Use the figure to sketch s. To print an enlarged copy of the graph, go to the website www.mathgraphs.com. (c) Find the magnitude of s. What information does the magnitude give you about the skydiver’s fall? (d) If there were no wind, the skydiver would fall in a path perpendicular to the ground. At what angle to the ground is the path of the skydiver when the skydiver is affected by the 40-mile-per-hour wind from due west? (e) The skydiver is blown to the west at 30 miles per hour. Draw a new figure that gives a visual representation of the problem and find the skydiver’s new velocity. 491 7. Write the vector w in terms of u and v, given that the terminal point of w bisects the line segment (see figure). v w When taking off, a pilot must decide how much of the thrust to apply to each component. The more the thrust is applied to the horizontal component, the faster the airplane will gain speed. The more the thrust is applied to the vertical component, the quicker the airplane will climb. u Lift Thrust 8. Prove that if u is orthogonal to v and w, then u is orthogonal to Climb angle θ cv dw for any scalars c and d (see figure). Velocity θ Drag Weight FIGURE FOR v w 10 (a) Complete the table for an airplane that has a speed of v 100 miles per hour. u 9. Two forces of the same magnitude F1 and F2 act at angles !1 and !2, respectively. Use a diagram to compare the work done by F1 with the work done by F2 in moving along the vector PQ if (a) !1 !2 (b) !1 60 and !2 30 . 10. Four basic forces are in action during flight: weight, lift, thrust, and drag. To fly through the air, an object must overcome its own weight. To do this, it must create an upward force called lift. To generate lift, a forward motion called thrust is needed. The thrust must be great enough to overcome air resistance, which is called drag. For a commercial jet aircraft, a quick climb is important to maximize efficiency because the performance of an aircraft at high altitudes is enhanced. In addition, it is necessary to clear obstacles such as buildings and mountains and to reduce noise in residential areas. In the diagram, the angle ! is called the climb angle. The velocity of the plane can be represented by a vector v with a vertical component v sin ! (called climb speed) and a horizontal component v cos !, where v is the speed of the plane. 492 ! 0.5 1.0 1.5 2.0 2.5 3.0 v sin ! v cos ! (b) Does an airplane’s speed equal the sum of the vertical and horizontal components of its velocity? If not, how could you find the speed of an airplane whose velocity components were known? (c) Use the result of part (b) to find the speed of an airplane with the given velocity components. (i) (ii) v sin ! 5.235 miles per hour v cos ! 149.909 miles per hour v sin ! 10.463 miles per hour v cos ! 149.634 miles per hour