f(x+y) + f(x-y) = c(x+y)^2 + c(x-y)^2 = c(x^2 + 2xy + y^2 + x^2 - 2xy + y^2) = c(2x^2 + 2y^2) = 2cx^2 + 2cy^2 = 2f(x) + 2f(y)