Question 591078
FOILing the left side ___ 1 - cos^4 = 2 sin^2 - sin^4


adding (sin^4 - 1) ___ sin^4 - cos^4 = 2 sin^2 - 1


factoring ___ (sin^2 + cos^2) (sin^2 - cos^2) = 2 sin^2 - (sin^2 + cos^2)


1 (sin^2 - cos^2) = sin^2 - cos^2


1 = 1