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更完整的內文
\begin{flushleft}
\Large{Strauss PDE Exercises 1.2.}
\end{flushleft}
\\
\begin{flushleft}
\begin{itemize}
\item[{\large 1.2.8.}]
Let \left\{ \begin{array}{c} x' = ax + by &
y' = bx - ay \end{array} \right.
$\Rightarrow$ \left\{ \begin{array}{c}
u_{x} = au_{x'} + bu_{y'} &
u_{y} = bu_{x'} - au_{y'}
\end{array}
\right. .\\
Therefore,
\begin{align}
& \text{ }au_{x}+bu_{y}+cu = a^{2} u_{x'} + b^{2} u_{x'}+cu =0
\notag \\
\Leftrightarrow & \text{ }u_{x'} = \frac{-c}{a^{2}+b^{2}} u
\notag \\
\Leftrightarrow & \text{ }u = C_{1}(y') \exp \left(\frac{-c}{a^{2}+b^{2}} x'
\right) =:
f(bx-ay) \exp \left(\frac{-c(ax+by)}{a^{2}+b^{2}}
\right) . \notag
\end{align} \\
\end{itemize}
\end{flushleft}
\end{document}
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