need help on this problem im lost on this one

Question

Use an Addition or Subtraction Formula to write the expression as a trigonometric function of one number.

sin 

4π

7

 cos 

5π

21

 − cos 

4π

7

 sin 

5π

21

 

 

 

        Find its exact value.

Answer 1

Step 1:

The trigonometric function is .

 

Fundamental trigonometric identities: .

Rewrite the trigonometric function is  .

Simplify the function.

Fundamental trigonometric identities: .

Solution:

The value of trigonometric function is .