y/(x＋y)²对x求偏导数，最好有过程
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z = y/(x+y)^2
∂z/∂x
= y∂/∂x [1/(x+y)^2]
=y[-2/(x+y)^3]
=-2y/(x+y)^3
∂z/∂y
= ∂/∂y [y/(x+y)^2]
= [(x+y)^2. ∂/∂y (y) - y∂/∂y(x+y)^2 ] /(x+y)^4
= [(x+y)^2 - 2y(x+y) ] /(x+y)^4
= [(x+y) - 2y ] /(x+y)^3
=(x-y)/(x+y)^3
∂z/∂x
= y∂/∂x [1/(x+y)^2]
=y[-2/(x+y)^3]
=-2y/(x+y)^3
∂z/∂y
= ∂/∂y [y/(x+y)^2]
= [(x+y)^2. ∂/∂y (y) - y∂/∂y(x+y)^2 ] /(x+y)^4
= [(x+y)^2 - 2y(x+y) ] /(x+y)^4
= [(x+y) - 2y ] /(x+y)^3
=(x-y)/(x+y)^3
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