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Pre-Calculus Section 6.2- Trigonometry of Right Triangles
Objective: SWBAT study several applications of the trigonometric ratios.
Homework: Day 1 Page 484 (1- 31) odd; Day 2 (24-28 even, 33, 35, 39, 41, 43, 45,
49,50, 51, 52)
Daily Warm-Up
1. Simplify (3x  5x 2  2 x3 )(6 x 2  5x  1)1
Trigonometric Ratios
Consider a right triangle with  as one of its acute angles. The trigonometric
ratios are defined as follows
The Trigonometric Ratios
opposite
hypotenuse
sin  
csc  
hypotenuse
opposite
adjacent
hypotenuse
cos =
sec =
hypotenuse
adjacent
opposite
adjacent
tan =
cot =
adjacent
opposite
hypotenuse
opposite

adjacent
The symbols we use for these ratios are abbreviations for their full names: sine, cosine,
tangent, cosecant, secant, cotangent. Since any two right triangles with angle  are
similar, these ratios are the same, regardless of the size of the triangle; the trigonometric
ratios depend only on the angle  .
Example 1: Finding Trigonometric Ratios.
Find the six trigonometric ratios of the angle  in the figure.
2
3

5
Example 2: Finding Trigonometric Ratios
3
If cos   , sketch a right triangle with acute angle  , and find the other five
4
trigonometric ratios of  .
Special Triangles
45o
2
1
3
30o
45o
60 o
1
1
 in
degrees
30o
45o
60 o
 in
radians
sin 
2
cos 
tan 
csc 
sec 
cot 
It is useful to remember these special trigonometric ratios because they occur often. To
find values of the trigonometric ratios for other angles, we use a calculator. Calculators
give the values for sine, cosine, and tangent; the other ratios can be easily calculated from
these using the following reciprocal relations:
csc t 
1
sin t
sec t 
1
cos t
cot t 
1
tan t
** Note: Make sure to set your calculator in radian mode if the angle is measured in
radians and degree mode if the angle is measured in degrees.
Example 3: Using a Calculator to Find Trigonometric Ratios
Find the approximate value for each angle correct to five decimal places.
a) sin17o
b) sec88o
c) cos1.2
d) cot1.54
Applications of Trigonometry of Right Triangles
A triangle has six parts: three angles and three sides. To solve a triangle means to
determine all of its parts from the information known about the triangle, that is, to
determine the lengths of the three sides and the measures of the three angles.
B
Example 4: Solving a Right Triangle
Solve triangle ABC, shown in the figure.
12
A
30
a
o
b
C
The ability to solve right triangles using the trigonometric ratios is fundamental to many
problems in navigation, surveying, astronomy, and the measurement of distances. The
applications we consider in this section always involve right triangles but, as we will see
in the next three sections, trigonometry is also useful in solving triangles that are not right
triangles.
To discuss the next examples, we need some terminology. If an observer is looking at an
object, then the line from the eye of the observer to the object is called the line of sight.
If the object being observed is above the horizontal, then the angle between the line of
sight and the horizontal is called the angle of elevation. If the object is below the
horizontal, then the angle between the line of sight and the horizontal is called the angle
of depression. In many of the examples and exercises in this chapter, angles of elevation
and depression will be given for a hypothetical observer at ground level. If the line of
sight follows a physical object, such as an inclined plane or a hillside, we use the term
angle of inclination.
Example 5: Finding the Height of a Tree
A giant redwood tree casts a shadow 532 ft long. Find the height of the tree if the
angle of elevation of the sun is 25.7 o .
Example 6: A Problem Involving Right Triangles
From a point on the ground 500 ft from the base of a building, an observer finds
that the angle of elevation to the top of he building is 24o and that the angle of elevation
to the top of a flagpole atop the building is 27 o . Find the height of the building and the
length of the flagpole.
In some problems we need to find an angle in a right triangle whose sides are given.
To find an angle we use the sin 1 or ARCSIN , cos 1 or ARCCOS , and
tan 1 or ARCTAN keys on a calculator.
Example 7: Solving for an Angle in a Right Triangle
A 40-ft ladder leans against a building. If the base of the ladder is 6ft from the
base of the building, what is the angle formed by the ladder and the building?
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