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\begin{equation*}
\frac{dy}{dx} = c_{1} e^{x} -3c_{2} e^{-3x}\qquad , \qquad \frac{d^2 y}{dx^2}=c_{1} e^{x}+9 c_{2} e^{-3x}
\Longrightarrow (c_{1} e^{x}-3 c_{2} e^{-3x})+(c_{1} e^{x}+9 c_{2} e^{-3x})-3(c_{1}e^{x}+c_{2} e^{-3x})\neq 0.
\end{equation*}
یا
\begin{align*}
\frac{dy}{dx} &= c_{1} e^{x} -3c_{2} e^{-3x}\\
\frac{d^2 y}{dx^2}&=c_{1} e^{x}+9 c_{2} e^{-3x} \\
&\Rightarrow (c_{1} e^{x}-3 c_{2} e^{-3x})+(c_{1} e^{x}+9 c_{2} e^{-3x})-3(c_{1}e^{x}+c_{2} e^{-3x})\neq 0.
\end{align*}
\end{document}
