Question

Prove that;cos(pi/4 - x) cos(pi/4 - y) - sin(pi/4 - x) sin(pi/4 - y) = sin(x+y)

Answer 1

we know that cosAcosB-sinAsinB=cos(A+B)
so from the above expression we get
cos(pi/2-(X+Y))
=sin(X+Y)