中三級數學問題(急!!)

Prove : (cos @)^4 - (sin @)^4 = 2(cos @)^2 - 1

By a), LHS = [ (cos @)^2 - (sin @)^2 ] [ (cos @)^2 + (sin @)^2]

= (cos @)^2 - (sin @)^2

By b) , (cos @)^2 - (sin @)^2

= 2(cos @)^2 - 1 = RHS

(Proved)