докажите тождество sin^4 a-cos^4 a=sin^2 a-cos^2 a

a^2-b^2 = (a-b) (a+b)sin^2a+cos^2a=1(sin^2-cos^2a)(sin^a+cos^2a) = sin^2a-cos^2a1(sin^2a-cos^2a) = sin^2a-cos^2asin^2a-cos^2a=sin^2a-cos^2a