Asked by lindsay

Simplify
cos Theta/4 cos theta/4 - sin theta/4 sin theta/4.
Answered by Reiny
recall
cos 2A = cos^2 A - sin^2 A
you have that pattern

cos(Ø/4)cos(Ø/4) - sin(Ø/4)sin(Ø/4)
= cos^2 (Ø/4) - sin^2 (Ø/4)
= cos (Ø/2)