Question

How do you express `cos( (5 pi)/4 ) * cos (( pi) /3 )` without using products of trigonometric functions?

Answer 1

Use cos(A+B) + cos(A-B) = 2 cos(A) cos(B).
The answer is `(cos(19pi/12) + cos(11pi/12))/2`

Explanation:

Here, `A = 5pi/4 and B = pi/3`