先求dz/dx
两边对x求偏导
2z*dz/dx-y+dz/dx=0
dz/dx=y/(2z+1)
再求dz/dy
同理
dz/dy=x/(2z+1)
然后
d^2z/dxdy=d/dx(dz/dy)=d/dx[x/(2z+1)]
dx/dx *(2z+1) - x*d(2z+1)/dx
= ----------------------------------------------
(2z+1)^2
(2z+1) - x*2*dz/dx
= ------------------------------
(2z+1)^2
(2z+1) - 2xy/(2z+1)
= -----------------------------------
(2z+1)^2
(2z+1)^2- 2xy
= ----------------------
(2z+1)^3
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