In this paper the Decomposition Theorem for functional equations is shown. As an application of this Theorem the two times continuously differentiable solution of the functional equation G 1 (x(x + y)) + F 1 (y) = G 2 (y(x + y)) + F 2 (y) can be given with unknown functions G i , F i : R + → R (i = 1, 2) where the Equation is fulfilled for all x, y ∈ R + (where R + := {x ∈ R | x > 0}).