On Approximation of Some Mixed Functional Equations

In this paper,  we have improved some of the results in [C. Choi and   B. Lee, Stability of Mixed Additive-Quadratic and Additive-Drygas Functional Equations. Results Math.  75  no. 1 (2020), Paper No. 38]. Indeed, we investigate the Hyers-Ulam stability problem of the following functional equations / 2φ(x + y) + φ(x - y) = 3φ(x)+ 3φ(y)  \  2ψ(x + y) + ψ(x - y) = 3ψ(x) + 2ψ(y) + ψ(-y). We also consider the Pexider type functional equation / 2ψ(x + y) + ψ(x - y) = f(x) + g(y), and the additive functional equation 2ψ(x + y) + ψ(x - y) = 3ψ(x) + ψ(y).