Question

How do you express `cos( 3 pi/ 2 ) * cos (( 17 pi) / 12 )` without using products of trigonometric functions?

Answer 1

see below

Explanation:

Use formula `2cosAcosB=cos(A+B)+cos(A-B)`

`cos((3pi)/2) cos ((17pi)/2)=1/2 [cos(((3pi)/2)+((17pi)/2)) + cos(((3pi)/2)-((17pi)/2))]`

`=1/2[cos10pi+cos(-7pi)]`

`=1/2 cos 10pi + 1/2 cos7pi`