In this paper, we introduce the following additive type functional equation f(rx+ sy) = r + s 2 f(x+ y) + r − s 2 f(x− y), where r, s ∈ R with r + s, r − s 6= 0. Also we investigate the Hyers–Ulam–Rassias stability of this functional equation in Banach modules over a unital C∗-algebra. These results are applied to investigate homomorphisms between C∗-algebras. 2000 Mathematics Subject Classific...