Approximate Additive Functional Equations in Closed Convex Cone

In this paper, we introduce the following positive-additive functional equation in C*-algebras f(x + 4(sic) + 6 root xy + 4(sic) + y) = f(x) + 4f(x)(3/4) (sic) + (6 root f(x)f(y) + 4f(y)(3/4) (sic) + f(y). Using the fixed point method, we prove the stability of the positive-additive functional equation in C*-algebras. Moreover, we prove the Hyers-Ulam stability of the above functional equation in C*-algebras by the direct method of Hyers-Ulam.