1. Right Triangle Trigonometry
Digital Lesson
Right Triangle Trigonometry

2. Trigonometric Functions
The six trigonometric functions of a right triangle, with an acute angle , are defined by ratios of two sides of the triangle.
The sides of the right triangle are: θ
hyp
 the side opposite the acute angle ,
opp
 the side adjacent to the acute angle ,
adj
 and the hypotenuse of the right triangle.
The trigonometric functions are
sine, cosine, tangent, cotangent, secant, and cosecant.
sin  = cos  = tan  =
csc  = sec  = cot  =
opp
hyp
adj
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Trigonometric Functions

3. Example: Six Trig Ratios
Calculate the trigonometric functions for  . 4 3 5 
The six trig ratios are
sin  =
cos  =
tan  =
cot  =
sec  =
csc  =
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Example: Six Trig Ratios

4. Geometry of the 45-45-90 Triangle
Consider an isosceles right triangle with two sides of length 1. 1 45
The Pythagorean Theorem implies that the hypotenuse is of length
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Geometry of the Triangle

5. Example: Trig Functions for  45
Calculate the trigonometric functions for a 45 angle. 1 45
sin 45 = = =
cos 45 = = =
hyp
adj
tan 45 = = = 1
adj
opp
cot 45 = = = 1
opp
adj
sec 45 = = =
adj
hyp
csc 45 = = =
opp
hyp
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Example: Trig Functions for  45

6. Geometry of the 30-60-90 Triangle2
Consider an equilateral triangle with each side of length 2.
60○
30○
30○
The three sides are equal, so the angles are equal; each is 60.
The perpendicular bisector of the base bisects the opposite angle. 1 1
Use the Pythagorean Theorem to find the length of the altitude,
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Geometry of the Triangle

7. Example: Trig Functions for  30
Calculate the trigonometric functions for a 30 angle. 1 2 30
cos 30 = =
hyp
adj
sin 30 = =
tan 30 = = =
adj
opp
cot 30 = = =
opp
adj
sec 30 = = =
adj
hyp
csc 30 = = = 2
opp
hyp
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Example: Trig Functions for  30

8. Example: Trig Functions for  60
Calculate the trigonometric functions for a 60 angle. 1 2
60○
sin 60 = =
cos 60 = =
hyp
adj
tan 60 = = =
adj
opp
cot 60 = = =
opp
adj
sec 60 = = = 2
adj
hyp
csc 60 = = =
opp
hyp
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Example: Trig Functions for  60

9. Example: Using Trigonometric Identities
Trigonometric Identities are trigonometric equations that hold for all values of the variables.
Example: sin  = cos(90  ), for 0 <  < 90
Note that  and 90  are complementary angles.
Side a is opposite θ and also adjacent to 90○– θ . a
hyp b θ
90○– θ
sin  = and cos (90  ) = .
So, sin  = cos (90  ).
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Example: Using Trigonometric Identities

10. Fundamental Trigonometric Identities for
Cofunction Identities
sin  = cos(90  ) cos  = sin(90  ) tan  = cot(90  ) cot  = tan(90  ) sec  = csc(90  ) csc  = sec(90  )
Reciprocal Identities
sin  = 1/csc  cos  = 1/sec  tan  = 1/cot  cot  = 1/tan  sec  = 1/cos  csc  = 1/sin 
Quotient Identities
tan  = sin  /cos  cot  = cos  /sin 
Pythagorean Identities
sin2  + cos2  = 1 tan2  + 1 = sec2  cot2  + 1 = csc2 
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Fundamental Trigonometric Identities for

11. Example: Given 1 trig function, find other functions
Example: Given sin  = 0.25, find cos , tan , and sec .
Draw a right triangle with acute angle , hypotenuse of length one, and opposite side of length 0.25.
Use the Pythagorean Theorem to solve for the third side. θ 1
0.25
cos  = = tan  = = sec  = = 1.033
0.9682
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Example: Given 1 trig function, find other functions

12. Example: Given 1 Trig Function, Find Other Functions
Example: Given sec  = 4, find the values of the other five trigonometric functions of  .
Draw a right triangle with an angle  such that 4 = sec  = = .
adj
hyp θ 4 1
Use the Pythagorean Theorem to solve for the third side of the triangle.
sin  = csc  = =
cos  = sec  = = 4
tan  = = cot  =
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Example: Given 1 Trig Function, Find Other Functions

13. Example: Given 1 trig function, find other functions
Example: Given sin  = 0.25, find cos , tan , and sec .
Draw a right triangle with acute angle , hypotenuse of length one, and opposite side of length 0.25.
Use the Pythagorean Theorem to solve for the third side. θ 1
0.25
cos  = = tan  = = sec  = = 1.033
0.9682
Copyright © by Houghton Mifflin Company, Inc. All rights reserved.
Example: Given 1 trig function, find other functions