x2 + x2 = 2x2. Don't be thrown by squares (or other powers).y + y = 2y. Just treat x2 like you treat y. But remember, you can only add together the same powers of x, you cannot add together x and x2...
sin2 x = (1/2)(1 - cos 2x)cos2 x = (1/2)(1 + cos 2x)Multiplying both you get (1/4) (1 - cos2 2x)Which is equal to (1/4) (1 - (1/2) (1 + cos 4x)= (1/8) (2 - 1 - cos 4x)= (1/8) (1 - cos 4x)