Question

Question #df465

Answer 1

`LHS=cos(3a+a)`

`=cos(2xx2a)`

`=2cos^2 2a-1`

`=2(2cos^2a-1)^2-1`

`=2(4cos^4a-4cos^2a+1)-1`

`=8cos^4a-8cos^2a+2-1`

`=1-8cos^2a+8cos^4a=RHS`

Proved