# These identities are on the formula sheet by Op8sxmt

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```									                            TRIGONOMETRIC IDENTITIES

1. Reciprocal, Quotient, and Pythagorean Identities – pg. 290-297
Assignment: pg. 296-297 #1-8, 10-14, 15
2. Sum, Difference, and Double-Angle Identities – pg. 299-308
Assignment: pg. 306-308 #1-11, 15-17, 20, 23
3. Proving Identities – pg. 309-315
Assignment: pg. 314-315 #1-8, 10-15, 16-18
4. Solving Trigonometric Equations Using Identities – pg. 316 – 321
Assignment: pg. 320-321 #1-11, 14-16, 17
5. Chapter Quiz
6. Chapter Review – pg. 322-323
Assignment: pg. 322-323 #1-21
7. Chapter Exam
LESSON 1: RECIPROCAL, QUOTIENT, AND PHYTHAGOREAN IDENTITIES
Learning Outcomes:
   Learn to verify a trigonometric identity numerically and graphically using technology
   Learn to explore reciprocal, quotient, and Pythagorean identities
   To determine non-permissible values of trigonometric identities
   To explain the difference between a trigonometric identity and a trigonometric equation

What is the difference between an equation and an identity?

is an equation. It is only true for certain values of the variable x. The solutions to
this equation are -2 and 2 which can be verified by substituting these values into the equation.
is an identity. It is true for all values of the variable x.

Comparing Two Trigonometric Expressions
1. Graph the curves y = sin x and y = cos x tan x over the domain                           Graph
the curves on separate grids using the same range and scale. What do you notice?

2. Use the table function on your calculator to compare the two graphs. Describe your findings.

3. Use your knowledge of tan x to simplify the expression cos x tan x

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4. What are the non-permissible values of x in the equation sin x = cos x tan x?

Given                 , the non-permissible values will occur when cos x = 0. That occurs at 90˚

and again at 270˚.

5. Are there any permissible values for x for which the expressions are not equal?

The equations y = sin x and y = cos x tan x are examples of trigonometric identities.
Trigonometric identities can be verified both numerically and graphically.

Reciprocal Identities:
1                         1                                 1
csc x                   sec x                             cot x 
sin x                     cos x                             tan x

Quotient Identities:
sin x                               cos x
tan x                              cot x 
cos x                               sin x

Verifying Identities for a Particular Case:
When verifying an identity we must treat the left side (LS) and the right side (RS) separately and
work until both sides represent the same value.
This technique does not prove that an identity is true for all values of the variable – only for the
value of the variable being verified.
sin x
Ex. Verify that tan x          .
cos x

a. Determine the non-permissible values, in degrees, for the equation.

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b. Verify that θ= 60˚.

L.S.                                            R.S.
sin 60
tan 60
cos 60
3
 3 (using exact value chart)                                        2
1
2
3 2
     
2 1
 3

LS = RS

c. Verify graphically that this is an identity.

Ex. a. Determine the non-permissible values, in radians, of the variable in the expression

The functions sec x and tan x both have non-permissible values in their domains:

The value of tan x also cannot equal 0. Tan θ =0 at π

Therefore, combine the restriction is            . A restriction will occur every 90˚.

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b. Simplify the expression.

Pythagorean Identity

Recall that point P on the terminal arm of an angle θ in standard position has coordinates (cos θ,

sin θ). Consider a right triangle with a hypotenuse of 1 and legs of cos θ and sin θ.

y
P(cosθ, sinθ)

x

The hypotenuse is 1 because it is the radius of the unit circle. Apply the Pythagorean Theorem
in the right triangle to establish the Pythagorean Identity:

Pythagorean Identities:

sin 2 x  cos2 x  1      1  tan 2 x  sec2 x     1  cot 2 x  csc2 x

These identities can be written in several ways:

sin 2 x  1  cos2 x               cos2 x  1  sin 2 x
tan 2 x  sec2 x  1               cot 2 x  csc2 x  1

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Ex. Verify the identity                       for the given values:

a.

L.S. =                                       R.S. =

b.

240˚(QIII) will have a ref angle of 60˚, therefore the Tan will be positive and sin will be

negative.

L.S. =                                            R.S. =

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b. Use quotient identities to verify the Pythagorean identity

Assignment: pg. 296-297 #1-8, 10-14, 15

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LESSON 2: SUM, DIFFERENCE, AND DOUBLE-ANGLE IDENTITIES

Learning Outcomes:

    Learn to apply sum, difference, and double-angle identities to verify the
equivalence of trigonometric expressions
    To verify a trigonometric identity numerically and graphically using technology

Ex. Use exact values to verify the following statement:
a. sin  60  30   sin 60 cos30  sin30 cos60

L.S. =

R.S. =

Sum and Difference Identities:

sin( A  B )  sin A cos B  cos A sin B
sin( A  B )  sin A cos B  cos A sin B
cos( A  B )  cos A cos B  sin A sin B
cos( A  B)  cos A cos B  sin A sin B
tan A  tan B
tan( A  B) 
1  tan A tan B
tan A  tan B
tan( A  B) 
1  tan A tan B
These are on the formula sheet

Ex. Write the expression                                        as a single trigonometric function.
Has the same form as the right side of cos(A-B):

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Ex. Find the exact value of sin15

break angle into exact values
sin15  sin  45  30   sin 45 cos30  sin30 cos 45

Ex. Find the exact value of

When faced with radians, it may be easier to switch to degrees (165˚)
Rewrite as a addition of two special angles (135 + 30)

From special triangles

For 135, it has a reference angle of 45 in QII (tan is negative). Tan 45 = 1

Simplify by combining fractions:

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Double Angle Identities:

sin 2 A  2sin A cos A
cos 2 A  cos 2 A  sin 2 A
2 tan A
tan 2 A 
1  tan 2 A

These identities are on the formula sheet

Ex. Express each in terms of a single trigonometric function:

a. 2 sin 4x cos 4x

1          1
b. cos 2     A  sin 2 A
2          2

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5       5
c. sin x cos x
2       2

1     5     5 
 2sin x cos x 
2     2     2 
1      5 
 sin 2  x 
2      2 
1
 sin 5 x
2
(the 0.5 is added to the outside of the brackets to balance off the 2 added to the inside, the 2 is
added to match the identity sin2A=2 sinAcosA)

Ex. Determine an identity for cos 2A that contains only the sine ratio.

Ex. Determine an identity for cos 2A that contains only the cosine ratio.

Ex. Simplify the expression           to one of the three primary trigonometric ratios

Assignment: pg. 306-308 #1-11, 15-17, 20, 23

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LESSON 3: PROVING IDENTITIES
Learning Outcomes:
   To prove trigonometric identities algebraically
   To understand the difference between verifying and proving an identity
   To show that verifying the two sides of a potential identity are equal for a given value is
insufficient to prove the identity

To prove that an identity is true for all permissible values, it is necessary to express both sides of
the identity in equivalent forms. One or both sides of the identity must be algebraically
manipulated into an equivalent form to match the other side.
You cannot perform operations across the equal sign when proving a potential identity. Simplify
the expressions on each side of the identity independently.

Hints in Proving an Identity:
1. Begin with the more complex side
2. If possible, use know identities given on the formula sheet, eg. try to use
the Pythagorean identities when squares of trigonometric functions are
involved.
3. If necessary change all trigonometric ratios to sines and/or cosines, eg.
sin x                 1
replace tan x by         , or sec x by       .
cos x               cos x
4. Look for factoring as a step in trying to prove an identity.
5. If there is a sum or difference of fractions, write as a single fraction
6. Occasionally, you may need to multiply the numerator or denominator of
a fraction by its conjugate.

It is usually easier to make a complicated expression simpler than it is to make a simple
expression more complicated.

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1
Ex. Consider the statement          cos x  sin x tan x
cos x

a. Verify the statement is true for x 
3
L.S.                                  R.S.

b. Prove the statement algebraically:

L.S.                                          R.S.

c. State the non-permissible values for x. Work in degrees.
The only non-permissible values for either side is cos x ≠ 0.

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Ex. Consider the identity

a. Describe how to use a graphing calculator to verify the identity.

b. Prove the identity

L.S.                                              R.S.

Ex. Prove that                  is an identity for all permissible values of x.

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Ex. Prove that                          is an identity. For what values of x is this identity

undefined?

For non-permissible values, we need to find we the equations cos x – 3 = 0 or 2cos x – 1 = 0 are

undefined.

No solution.
Ref angle is 60˚
QI: 60˚
QIV: 360˚-60˚ = 300˚
Undefined for
x = 60˚+360˚n
x = 300˚+360˚n

Assignment: pg. 314-315 #1-8, 10-15, 16-18

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LESSON 4: SOLVING TRIGONOMETRIC EQUATIONS USING IDENTITIES

Learning Outcomes:

     To solve trigonometric equations algebraically using know identities
     To determine exact solutions for trigonometric equations where possible
     To determine the general solution for trigonometric equations
     To identify and correct errors in a solution for a trigonometric equation

Investigation
1. Graph the function                       over the domain                       . Describe the
graph.

2. From the graph, determine an expression for the zeros of the function over the domain of all
real numbers.

question above.

Ref angle is 60˚(answers in QI and QIV)

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4. Which method, graphic or algebraic, do you prefer to solve the above equation?

To solve some trigonometric equations, you need to make substitutions using the trigonometric
identities that you have studied in this chapter. This often involves ensuring that the equation is
expressed in terms of one trigonometric function.

Ex. Solve the following equation where 0  x  2 .

2cos2 x  3sin x  0

2(1  sin 2 x)  3sin x  0  Pythagorean identity
2  2sin 2 x  3sin x  0  Expand brackets
0  2sin 2 x  3sin x  2  Need to factor
 2sin x 1sin x  2

1
sin x                                    sin x  2
2

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Ex. Solve for x as an exact value where

Ex. Solve the equation                        algebraically over the domain

Check for any non-permissible values: occur when cos x = 0, at 90˚, 270˚

The non-permissible values are not part of our solution set, therefore our solutions are x = 30˚,

150˚, and 180˚

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Ex. Algebraically solve                 . Give general solutions expressed in radians.

Occurs at 0
Ref angle is , neg in QII andQIII

General Solutions:

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Ex. Algebraically solve                         Give general solutions expressed in radians.

Occurs at 0
No solution.

Only non-permissible value occurs when cosx = 0, (90˚, 270˚)

The non-permissible values do not change our original solution of 0 +2πn

Unless the domain is restricted, give general solutions. Check that the solutions for an equation

do not include non-permissible values from the original equation.

Assignment: pg. 320-321 #1-11, 14-16, 17

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