prove that..
cos4A=1-8cos2a+8cos4a
Hi!
Here is the answer to your question.
L.H.S. = cos 4A
= cos 2(2A)
= 1 – 2 sin2 2A
= 1 – 2(2 sin A cos A)2
= 1 – 8 sin2A cos2A
= 1 – 8 (1 – cos2A) cos2A [1 – cos2A = sin2A]
= 1 – 8 cos2A + 8cos4A
= R.H.S.
Hope you got the proof.
Cheers!