= sin^2 (pi/4)cos^2(a) - cos^2(pi/4)sin^2(a)= (1/2)^2(cos^2(a)) - (1/2)^2(sin^2(a))= 1/4(cos^2(a) - sin^2(a))= 1/4(cos(2a))
= sin^2 (pi/4)cos^2(a) - cos^2(pi/4)sin^2(a)
= (1/2)^2(cos^2(a)) - (1/2)^2(sin^2(a))
= 1/4(cos^2(a) - sin^2(a))
= 1/4(cos(2a))