u=xy＋ yz ＋zx的所有二阶偏导数

U = xy + yz + xz
U'x = y + z.........U''xx = 0
U'y = x + z.........U''yy = 0
U'z = x + y.........U''zz = 0
U''xy = 1
U''yz = 1
U''zx = 1本回答被网友采纳

具体步骤如下：追答

追问

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