• 试题 ID 21910


已知函数 $f(x, y)=\frac{x^2+y^2}{x^2+y^2-x y}$, 则 $x \frac{\partial f(x, y)}{\partial x}+y \frac{\partial f(x, y)}{\partial y}=$
A 0.
B 1 .
C 2.
D 3 .
E 4
F
答案:

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解析:

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