Question

cos^2(2a)-sin^2(a)=cos(a)cos3a

Answer 1

by taking L.h.s.

cos ^2(2a) - sin^2 a

cos ^2(2a) - ( 1-cos 2a) /2

(cos 4a+1)/2 + (cos 2a-1)/2

1/2 [ 2 cos 3a cos a]

= cos 3a cos a

R.h.s