Answer:
here are the steps for the proof
Step-by-step explanation:
if u don't get it ask me on the comment section:)
L.H.S
=cos (3x)
=cos (x+2x)
=cosxcos2x - sinxsin2x
=cosxcos2x - 2sin^2xcosx
=cosx [cos2x - 2sin^2x]
=cosx [1-2sin^x-2sin^x]
=cosx [ 1-4sin^2x]
=R.H.S
