prove csc^4x-2csc^2x+1=cot^4x
csc^4 x - 2 csc² x + 1 == 1 / sin^4 x - 2 / sin² x + 1 = = ( 1 - 2 sin² x + sin^4 x ) / sin^4 x == ( sin² x - 1 )² / sin^4 x == ( cos² x )² / sin^4 x = cos^4 x / sin^ 4 x = cot ^4 x