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December 2, 2015 5.1 Angles of Triangles Geometry 5.1 Angles of Triangles Essential Question How are the angle measures of a triangle related? December 2, 2015 5.1 Angles of Triangles Goals – Day 1 Classify triangles by their sides Classify triangles by their angles Identify parts of triangles. Find angle measures in triangles. December 2, 2015 5.1 Angles of Triangles Triangle Symbol Use the picture for triangle. December 2, 2015 5.1 Angles of Triangles Triangle A triangle is a figure formed by three segments joining three noncollinear points. B C A This is ABC, which can also be named BCA, CAB, BAC, CBA, or ACB. December 2, 2015 5.1 Angles of Triangles Classifying Triangles by Sides Equilateral Isosceles Scalene December 3, 2015 5.1 Angles of Triangles Equilateral Triangle Three congruent sides. December 2, 2015 5.1 Angles of Triangles Isosceles Triangle At least two congruent sides. December 2, 2015 5.1 Angles of Triangles Scalene Triangle No congruent sides. December 2, 2015 5.1 Angles of Triangles Classifying Triangles by Angles Acute Equiangular Right Obtuse December 3, 2015 5.1 Angles of Triangles Acute Triangle Three acute angles December 2, 2015 5.1 Angles of Triangles Equiangular Triangle Three Congruent Angles December 2, 2015 5.1 Angles of Triangles Right Triangle One Right Angle December 2, 2015 5.1 Angles of Triangles Obtuse Triangle One Obtuse Angle December 2, 2015 5.1 Angles of Triangles And to add to the confusion… An equilateral triangle is also equiangular. An equiangular triangle is also acute. An equilateral can be considered an isosceles triangle. An equilateral triangle is also acute. December 2, 2015 5.1 Angles of Triangles Vertex Each of the three points joining the sides of a triangle is a vertex. There are three vertices in each triangle. Points A, B, and C are the vertices. B C A December 2, 2015 5.1 Angles of Triangles Adjacent Sides Two sides that share a common vertex are adjacent sides. The third side is the opposite side. A In RAT, RA and RT are adjacent sides. AT is the opposite side from ∠𝑅. R December 2, 2015 T 5.1 Angles of Triangles Isosceles Triangles (In this case, we consider an isosceles triangle with only two congruent sides.) The congruent sides are the LEGS. The third side is the BASE. Leg Leg Base December 2, 2015 5.1 Angles of Triangles Right Triangle The LEGS form the right angle. The third side (opposite the right angle) is the Hypotenuse. Leg Leg December 2, 2015 5.1 Angles of Triangles Hypotenuse From the Greek “stretched against”. Always longer than either leg. December 2, 2015 5.1 Angles of Triangles What have you learned so far? In the figure, 𝑀𝑁 ⊥ 𝑄𝑃 and 𝑀𝑃 ≅ 𝑀𝑄. Complete the following sentence. P 1. Name the legs of the isosceles triangle PMQ. N Segments PM and QM. Q December 2, 2015 5.1 Angles of Triangles M What have you learned so far? In the figure, 𝑀𝑁 ⊥ 𝑄𝑃 and 𝑀𝑃 ≅ 𝑀𝑄. Complete the following sentence. P 2. Name the base of isosceles triangle PMQ. N Segment PQ. Q December 2, 2015 5.1 Angles of Triangles M What have you learned so far? In the figure, 𝑀𝑁 ⊥ 𝑄𝑃 and 𝑀𝑃 ≅ 𝑀𝑄. Complete the following sentence. P 3. Name the hypotenuse of right triangle PNM. N Segment PM. Q December 2, 2015 5.1 Angles of Triangles M What have you learned so far? In the figure, 𝑀𝑁 ⊥ 𝑄𝑃 and 𝑀𝑃 ≅ 𝑀𝑄. Complete the following sentence. P 4. Name the legs of right triangle PNM. N Segments NP and NM. December 2, 2015 5.1 Angles of Triangles Q M What have you learned so far? In the figure, 𝑀𝑁 ⊥ 𝑄𝑃 and 𝑀𝑃 ≅ 𝑀𝑄. Complete the following sentence. P 5. Name the acute angles of right triangle QNM. N Q and NMQ Q December 2, 2015 5.1 Angles of Triangles M Example 1 Classify these triangles by its angles and by its sides. a. c. b. 125° Right , Scalene December 3, 2015 Obtuse , Equiangular, Equilateral Isosceles Isosceles , Acute 5.1 Angles of Triangles Example 2 Complete the sentence with always, sometimes, or never. Sometimes a right a. An isosceles triangle is ________ triangle. b. An obtuse triangle is ________ a right triangle. Never c. A right triangle is ________ an equilateral Never triangle. Sometimes an isosceles d. A right triangle is ________ triangle. December 2, 2015 5.1 Angles of Triangles Important Triangle Theorems 5.1 Triangle Sum Theorem 5.2 Exterior Angle Theorem December 2, 2015 5.1 Angles of Triangles 5.1 Triangle Sum Theorem The sum of the measures of the interior angles of a triangle is 180°. B mA + mB + mC = 180° A December 2, 2015 C 5.1 Angles of Triangles A Proof of the Triangle Sum Thm B Given: ABC Prove: 4 3 5 m1 + m2 + m3 = 180° A Statements 1. ABC 2. Draw line through point B parallel to AC 3. m4 + m3 + m5 = 180 1 Reasons 2 C 1. Given 2. Parallel Postulate (3.1) 4. m1 = m4 and m2 = m5 3. Def. of Straight Angle 4. Alternate Interior ’s 5. m1 + m3 + m2 = 180 5. Substitution December 2, 2015 5.1 Angles of Triangles Example 3 Find the measure of 1. Solution: 1 m1 + 70 + 32 = 180 m1 + 102 = 180 70° m1 = 180 – 102 m1 = 78° December 2, 2015 5.1 Angles of Triangles 32° Example 4 In MAD: mM = (2x)° = 2(20) = 40° mA = (3x)° = 3(20) = 60° mD = (4x) = 4(20) = 80° Find the measure of each angle, and classify. Solution: 2x + 3x + 4x = 180 This triangle is acute. 9x = 180 x = 20 December 2, 2015 5.1 Angles of Triangles Example 5 In RST: mR=(5x + 10) mS=(2x + 15) mT=(3x + 35) Find the measure of the three angles and then classify the triangle by angles. December 2, 2015 5.1 Angles of Triangles Example 5 Solution ACUTE (5x + 10) + (2x + 15) + (3x + 35) = 180 10x + 60 = 180 10x = 120 x = 12 mR=(5x + 10) = 5(12) + 10 = 70 mS=(2x + 15) = 2(12) + 15 = 39 mT=(3x + 35) = 3(12) + 35 = 71 December 2, 2015 5.1 Angles of Triangles Your Turn In ABC: mA=(x + 30) mB=x mC=(x + 60) Find the measure of the three angles and then classify the triangle by angles. December 2, 2015 5.1 Angles of Triangles Your Turn Solution In ABC: mA=(x + 30) mB=x mC=(x + 60). 𝑥 + 30 + 𝑥 + 𝑥 + 60 = 180 3𝑥 + 90 = 180 3𝑥 = 90 x = 30 m∠𝐴 = 30 + 30 = 60° m∠𝐵 = 30° m∠𝐶 = 30 + 60 = 90° December 2, 2015 5.1 Angles of Triangles RIGHT Your Turn Again. In ABC: mA=(6x + 11) mB=(3x + 2) mC=(5x - 1) Find the measure of the three angles and then classify the triangle by angles. December 2, 2015 5.1 Angles of Triangles Your Turn Again Solution 6𝑥 + 11 + (3𝑥 + 2) + 5𝑥 − 1 = 180 In ABC: mA=(6x + 11) 14𝑥 + 12 = 180 mB=(3x + 2) 14𝑥 = 168 mC=(5x - 1). x = 12 m∠𝐴 = 6(12) + 11 = 83° m∠𝐵 = 3 12 + 2 = 38° m∠𝐶 = 5 12 − 1 = 59° December 2, 2015 5.1 Angles of Triangles ACUTE Assignment December 2, 2015 5.1 Angles of Triangles December 2, 2015 5.1 Angles of Triangles Geometry 5.1 Angles of Triangles Essential Question How are the angle measures of a triangle related? December 2, 2015 5.1 Angles of Triangles 5.1 Day 2 Yesterday: The Interior Angle Theorem: the sum of the interior angles of a triangle is 180°. Today: The Exterior Angle Theorem December 2, 2015 5.1 Angles of Triangles But First… A corollary to the interior angle theorem. A corollary is a theorem that can be proved easily from another theorem. Not “big” enough to warrant title of theorem. A corollary follows from a theorem. December 2, 2015 5.1 Angles of Triangles Corollary to Theorem 5.1 The acute angles of a right triangle are complementary. m1 + m2 + 90 = 180 1 m1 + m2 = 90 QED 2 December 2, 2015 5.1 Angles of Triangles Example 1 Find X x = 70° 20° Since this is a right triangle, the acute angles are complementary, and 90 – 20 = 70. x° December 2, 2015 5.1 Angles of Triangles Interior and Exterior Angles Start with a triangle… December 2, 2015 5.1 Angles of Triangles Extend the sides…. 2 1 3 1, 2, 3 are INTERIOR ANGLES. They are INSIDE the triangle. December 2, 2015 5.1 Angles of Triangles 9 8 2 1 4 3 6 10 12 4, 6, 8, 9, 10, and 12 are EXTERIOR ANGLES. They are OUTSIDE the triangle. They are ADJACENT to the interior angles. December 2, 2015 5.1 Angles of Triangles 7 2 1 3 5 11 5, 7, and 11 are NOT EXTERIOR ANGLES. They are simply vertical angles to the interior angles. December 2, 2015 5.1 Angles of Triangles It is common (and less confusing) to draw only one exterior angle at a vertex. Exterior angles are always supplementary to the interior angles. 6 3 1 2 5 4 Interior Angles: 1, 2, 3 Exterior Angles: 4, 5, 6 December 2, 2015 5.1 Angles of Triangles 5.2 Exterior Angle Theorem The measure of an exterior angle of a triangle is equal to the sum of the measures of the two nonadjacent interior angles. 3 2 1 m1 = m2 + m3 December 2, 2015 5.1 Angles of Triangles Note: Sometimes (usually) the two nonadjacent interior angles are referred to as REMOTE INTERIOR ANGLES. The theorem then reads: An exterior angle of a triangle is equal to the sum of the two remote interior angles. December 2, 2015 5.1 Angles of Triangles 5.2 Exterior Angle Thm Proof (Informal) m2 + m3 + m4 = 180 ( angle sum) m4 + m1 = 180 (linear pair postulate) m2 + m3 + m4 = m4 + m1 (substitution) m2 + m3 = m1 (subtraction) 3 2 December 2, 2015 4 5.1 Angles of Triangles 1 Naming Remote Interior Angles 4 1 2 3 December 2, 2015 5 For exterior 1, the remote interior angles 6 & 8 are_____________. 6 8 7 9 5.1 Angles of Triangles Naming Remote Interior Angles 4 1 2 3 December 2, 2015 5 For exterior 4, the remote interior angles 2 & 8 are_____________. 6 8 7 9 5.1 Angles of Triangles Naming Remote Interior Angles 4 1 2 3 December 2, 2015 5 For exterior 5, the remote interior angles 2 & 8 are_____________. 6 8 7 9 5.1 Angles of Triangles Naming Remote Interior Angles 4 1 2 3 December 2, 2015 5 For exterior 9, the remote interior angles 2 & 6 are_____________. 6 8 7 9 5.1 Angles of Triangles Naming Remote Interior Angles 4 1 2 3 December 2, 2015 5 For remote interior angles 6 & 8, the exterior angle 1 or 3 is _____________. 6 8 7 9 5.1 Angles of Triangles Naming Remote Interior Angles 4 1 2 3 December 2, 2015 5 For remote interior angles 2 & 6, the exterior angle 7 or 9 is _____________. 6 8 7 9 5.1 Angles of Triangles Naming Remote Interior Angles 4 1 2 3 December 2, 2015 5 For remote interior angles 2 & 8, the exterior angle 4 or 5 is _____________. 6 8 7 9 5.1 Angles of Triangles Example 2 Find m1. 45° By Theorem 5.2: m1 + 45 = 110 1 December 2, 2015 110° m1 = 110 – 45 = 65° 5.1 Angles of Triangles Example 3 (x + 15) + 45 = 3x – 10 45° x + 60 = 3x – 10 70 = 2x (x + 15)° (3x – 10)° x = 35 Solve for x. December 2, 2015 5.1 Angles of Triangles Problems for You Use the exterior angle theorem! Write down the equation for each problem and solve. December 2, 2015 5.1 Angles of Triangles Your Turn. 1. Find m1 Solution: m1 = 32 + 125 m1 = 157 32 1 125 December 2, 2015 5.1 Angles of Triangles Solution: 2. Find m2 m2 + 45 = 165 m2 = 120 45 2 December 2, 2015 165 5.1 Angles of Triangles 3. Solve for x. Solution: 2x + 30 + 60 = 110 110° 2x + 90 = 110 2x = 20 (2x + 30)° 60 x = 10 December 2, 2015 5.1 Angles of Triangles Solution: 4. Solve for x. 12x – 4 = (6x + 8) + 5x 12x – 4 = 11x + 8 x = 12 (6x + 8) (5x) December 2, 2015 (12x – 4) 5.1 Angles of Triangles Solution: 5. Solve for x. (3x + 2) + (5x – 10) = 7x + 3 8x – 8 = 7x + 3 (3x + 2) x = 11 (7x + 3) (5x – 10) December 2, 2015 5.1 Angles of Triangles A Final Challenge Problem… Find the measure of each numbered angle. 1 50° 60° 4 60° 40° 5 60° 90° 2 60° 3 20° 30° December 2, 2015 6 100° 7 60° 5.1 Angles of Triangles Summary The sum of the interior angles of a triangle is 180 degrees. The acute angles of a right triangle are complementary. An exterior angle is equal to the sum of the two remote interior angles. December 2, 2015 5.1 Angles of Triangles Assignment December 2, 2015 5.1 Angles of Triangles