求数学高手解答！！求偏导数的微积分小题。。。在线等
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z = arctan[(x+y)/(1-xy)]
z'<x> = { [(1-xy)-(x+y)(-y)]/(1-xy)^2 } / {1+[(x+y)/(1-xy)]^2}
= (1+y^2)/[(1-xy)^2+(x+y)^2] = (1+y^2)/(1+x^2+y^2+(xy)^2]
= (1+y^2)/[(1+x^2)(1++y^2)] = 1/(1+x^2),
z''<xy>=0.
z'<x> = { [(1-xy)-(x+y)(-y)]/(1-xy)^2 } / {1+[(x+y)/(1-xy)]^2}
= (1+y^2)/[(1-xy)^2+(x+y)^2] = (1+y^2)/(1+x^2+y^2+(xy)^2]
= (1+y^2)/[(1+x^2)(1++y^2)] = 1/(1+x^2),
z''<xy>=0.
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