设z=xf(x+y)+g(xy,x2+y2),其中f,g分别二阶连续可导和二阶连续可偏导,则=________.

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问题 设z=xf(x+y)+g(xy,x2+y2),其中f,g分别二阶连续可导和二阶连续可偏导,则=________.

选项

答案f’+xf"+xy-1g’1+yxy-1lnxg’1+yx2y-1lnxg"11+2y2xy-1g"12+2xy+1lnxg"21+4xyg"22

解析 由z=xf(x+y)+g(xy,x2+y2)得
=f(x+y)+xf’(x+y)+yxy-1g’1(xy,x2+y2)+2xg’2(xy,x2+y2)
=f’+xf"+xy-1g’1+yxy-1lnxg’1+yx2y-1lnxg"11+2y2xy-1g"12+2xy+1lnxg"21+4xyg"22
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