Question

Evaluate = cos4A _ cos2A = sin4A _ sin2A

Answer 1

Cos 4 A - Cos2A

= (cos2A)2 - (cosA)2

= (1- sin2A)2 - cos2A

= 1 + sin4A - 2sin2A  -( 1- sin2A)

=1+ sin4A - 2 sin2A -1 + sin2A

=sin4A - sin2A

since LHS = RHS 

hence proved.