Giải phương trình: 4.sin^5x.cosx - 4.cos^5x.sinx = cos^2 4x + 1

4 sin^5x cos x - 4 cos^5x sinx = cos^2 4x +1
cos4x = cos^2 3x - cos^2 x + 1
4(sin^6x +cos^6x) - cos ( π/2 - 2x ) = 0
sin4x + cos^4x = cos2x
cos4x  = sin^2 x - 1/4