We prove the Hyers–Ulam stability of the functional equation $$\begin{aligned}&f(a_1x_1+a_2x_2,b_1y_1+b_2y_2)=C_{1}f(x_1,y_1)\nonumber \\ \nonumber \\&\quad +C_{2}f(x_1,y_2)+C_{3}f(x_2,y_1)+C_{4}f(x_2,y_2) \end{aligned}$$
f
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a
1
x
1
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a
2
x
2
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b
1
y
1
+
b
2
y
2
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=
C
1
f
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x
1
,
y
1
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+
C
2
f
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x
1
,
y
2
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+
C
3
f
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x
2
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y
1
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+
C
4
f
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x
2
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y
2
)
in the class of functions from a real or complex linear space into a Banach space over the same field. We also study, using the fixed point method, the generalized stability of $$(*)$$
(
∗
)
in the same class of functions. Our results generalize some known outcomes.