## Precalculus: Sum and Difference Identities Practice Problems ...

Precalculus: Sum and Difference Identities Practice Problems. Questions. 1. Find the value of sin. (. − π. 12. ) exactly. 2. Prove the identity cos. ( θ + π. 2. ).

Precalculus: Sum and Difference Identities Practice Problems

Questions  π exactly. 1. Find the value of sin − 12  π = − sin θ. 2. Prove the identity cos θ + 2 3. Prove the identity sin 4x + sin 2x = 2 sin 3x cos x. 

 5π 4. Find the value of sin − exactly by using the sine of a sum identity. This problem shows you a method to 12 determine exactly the trig functions at angles other than the special angles on the unit circle.

Page 1 of 4

Precalculus: Sum and Difference Identities Practice Problems

Solutions  π exactly. 1. Find the value of sin − 12 First, we need to figure out how to relate −π/12 to some of our special angles, since we are told to find this answer exactly. −π −2π 4π − 6π π π = = = − . 12 24 24 6 4 Therefore,  π sin − 12

= = = = =

√ hyp= 2

π − 6 4   π π π π sin cos − cos sin , use sin(u − v) = sin u cos v − cos u sin v 6 4 6 4 √ !     1 1 1 3 √ √ − , using reference triangles below 2 2 2 2 √ 1 3 √ − √ 2 2 2 2 √ 1− 3 √ 2 2 sin

opp=1

hyp=2

opp=1

 π 2. Prove the identity cos θ + = − sin θ. 2  π cos θ + 2

π

cos

cos θ − sin

π

sin θ, using cos(u + v) = cos u cos v − sin u sin v 2 2 = (0) cos θ − (1) sin θ, using the unit circle to evaluate the trig functions of pi/2. = − sin θ =

Page 2 of 4

Precalculus: Sum and Difference Identities Practice Problems

3. Prove the identity sin 4x + sin 2x = 2 sin 3x cos x.

sin 4x + sin 2x = = = = = = = = = =

sin(3x + x) + sin(x + x), (sin 3x cos x + cos 3x sin x) + (sin x cos x + cos x sin x) , use sin(u + v) = sin u cos v + cos u sin v twice. sin 3x cos x + (cos 3x) sin x + 2 sin x cos x, sin 3x cos x + (cos(2x + x)) sin x + 2 sin x cos x sin 3x cos x + (cos 2x cos x − sin 2x sin x) sin x + 2 sin x cos x, using cos(u + v) = cos u cos v − sin u sin v sin 3x cos x + cos 2x cos x sin x − sin 2x sin2 x + 2 sin x cos x sin 3x cos x + (cos 2x) cos x sin x − (sin 2x) sin2 x + 2 sin x cos x sin 3x cos x + (cos(x + x)) cos x sin x − (sin(x + x)) sin2 x + 2 sin x cos x sin 3x cos x + (cos x cos x − sin x sin x) cos x sin x − (sin x cos x + cos x sin x) sin2 x + 2 sin x cos x sin 3x cos x + cos3 x sin x − 3 sin3 x cos x + 2 sin x cos x

We can see that this is very nasty looking. Let’s start over. sin 4x + sin 2x = sin(3x + x) + sin(3x − x), = (sin 3x cos x + cos 3x sin x) + (sin 3x cos x − cos 3x sin x) , = 2 sin 3x cos x

use sin(u ± v) = sin u cos v ± cos u sin v

That was certainly easier! In this case, we exploited some of the hidden symmetry in the problem by writing 2x = 3x − 1 rather than 2x = x + x.

Page 3 of 4

Precalculus: Sum and Difference Identities Practice Problems



 5π 4. Find the value of sin − exactly by using the sine of a sum identity. This problem shows you a method to 12 determine exactly the trig functions at angles other than the special angles on the unit circle. First, we need to figure out how to relate −5π/12 to some of our special angles, since we are told to find this answer exactly. We are told to use a sum formula, so the sum of two of our special angles should produce −5π/12. −5π −10π −6π − 4π π π = = =− − . 12 24 24 4 6 Therefore,   5π sin − 12

= = = = = =