Question

How do you express `cos( (15 pi)/ 8 ) * cos (( 13 pi) /8 )` without using products of trigonometric functions?

Answer 1

((cos (7`pi`/2) + cos(`pi`/4))/2 = `sqrt`2 / 4

Explanation:

Use cos A . cos B = ( cos(A+B) + cos(A`-`B) )/2