Document Type : Research Paper

**Authors**

Department of Mathematics, Faculty of Sciences, University of Mohaghegh Ardabili, Ardabil, Iran.

**Abstract**

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

\begin{align*}

2\varphi(x + y) + \varphi(x - y) &= 3\varphi(x)+ 3\varphi(y) \\

2\psi(x + y) + \psi(x - y) &= 3\psi(x) + 2\psi(y) + \psi(-y).

\end{align*}

We also consider the Pexider type functional equation \[2\psi(x + y) + \psi(x - y) = f(x) + g(y),\] and the additive functional equation

\[2\psi(x + y) + \psi(x - y) = 3\psi(x) + \psi(y).\]

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