buktikan identitas trigonometri dari soal berikut... Cos^4-sin^4=1-2sin

cos^4 x - sin^4 x
= (cos^2 x + sin^2 x)(cos^2 x - sin^2 x)
= (1) ((1 - sin^2 x) - sin^2 x)
= 1 - 2 sin^2 x