Express cos(2x) in terms of acos^2(x) + b

cos(2x)=cos²(x) - sin²(x)use sin²(x) = 1 - cos²(x)so cos(2x)=cos²(x) - (1 - cos²(x))cos(2x)=2cos²(x) - 1so a = 2 and b = -1