((sin x + cos x)²)' = 2(sin x + cos x) * (sin x + cos x)' = 2(sin x + cos x)(cos x - sin x) = 2(cos²x - sin²x) = 2cos2x
((sin x + cos x)²)' = 2(sin x + cos x) * (sin x + cos x)' = 2(sin x + cos x)(cos x - sin x) = 2(cos²x - sin²x) = 2cos2x