例1
\left\{\!\!\begin{array}{l}
u(x, y, t) = -2 \cos \left( \frac{\pi t}{T} \right) \sin^2(\pi x) \cos(\pi y) \sin(\pi y) \\[2em]
v(x, y, z) = 2 \cos \left( \frac{\pi t}{T} \right) \cos(\pi x) \sin(\pi x) \sin^2(\pi y)
\end{array}\right.
\quad (T = 8)
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