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On the solution and stability of the quadratic type functional equations. (English) Zbl 1101.39307
Summary: We prove the solution and stability of the quadratic type functional equations with two variables \[ 4f(\frac{x-2y}{2})+f(x)+2f(y)=8f(\frac{x-y}{2})+f(2y) \] and \[ f(x-2y) + f(x)+2f(y)=2f(x-y)+f(2y). \]

MSC:
39B82 Stability, separation, extension, and related topics for functional equations
39B52 Functional equations for functions with more general domains and/or ranges
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