验证在整个Oxy平面内(x2+2xy+y2)dx+(x2+2xy一y2)dy是某个二元函数u(x,y)的全微分,并求这样的一个u(x,y).

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问题 验证在整个Oxy平面内(x2+2xy+y2)dx+(x2+2xy一y2)dy是某个二元函数u(x,y)的全微分,并求这样的一个u(x,y).

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答案令P(x,y)=x2+2xy+y2,Q(x,y)=x2+2xy—y2, [*] 所以(x2+2xy+y2)dx+(x2+2xy—y2)dy是某个二元函数u(x,y)的全微分. 故u(x,y)=∫(0,0)(x,y)(x2+2xy+y2)dx+(x2+2xy—y2)dy =∫0xx2dx+∫0y(x2+2xy-y2)dy =[*]

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