University of MaraghehSahand Communications in Mathematical Analysis2322-5807Articles in Press20201101On the Stability of Mixed Additive--Quadratic and Additive--Drygas Functional Equations4666510.22130/scma.2020.127585.801ENAbbasNajatiDepartment of Mathematics, Faculty of Sciences, University of Mohaghegh Ardabili, Ardabil, Iran.BatoolNooriDepartment of Mathematics, Faculty of Sciences, University of Mohaghegh Ardabili, Ardabil, Iran.Mohammad BagherMoghimiDepartment of Mathematics, Faculty of Sciences, University of Mohaghegh Ardabili, Ardabil, Iran.Journal Article20200628In 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<br />begin{align*}<br /> 2varphi(x + y) + varphi(x - y) &= 3varphi(x)+ 3varphi(y) \<br /> 2psi(x + y) + psi(x - y) &= 3psi(x) + 2psi(y) + psi(-y).<br />end{align*}<br />We also consider the Pexider type functional equation [2psi(x + y) + psi(x - y) = f(x) + g(y),] and the additive functional equation<br />[2psi(x + y) + psi(x - y) = 3psi(x) + psi(y).]