pembuktian identitas trigonometri sin⁴a-cos⁴a = 1-2 cos²a

sin^4 a - cos^4 a = 1 - 2 cos^2 a
(sin^2 + cos^2) (sin^2 a - cos^2 a) = 1 - 2 cos^2 a
sin^2 a - cos^2 a = 1 - 2 cos^2 a
(1 - cos^2)-cos^2 = 1 - 2 cos^2 a (Terbukti)