• 试题 ID 11903


设 $f(x, y)=\left\{\begin{array}{l}\frac{x^2 y}{\sqrt{x^2+y^2}}, x^2+y^2 \neq 0 \\ 0 \quad, x^2+y^2=0\end{array}\right.$ ,求二阶偏导数 $f_{x y}^{\prime \prime}$.
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