got 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 .

 

LCM of the trigonometric function is .

Addition of trigonometric identities: .

Solution :

The value of trigonometric function is .