Z^2*ln（X+Z)=XY
兩邊對(duì)X求微分 2Zln（X+Z)dZ/dX+(1+dZ/dX)Z^2/(X+Z)=Y
dZ/dX=(Y-Z^2/(X+Z))/（2Zln（X+Z)+Z^2/(X+Z)）
兩邊對(duì)Y求微分
2Zln（X+Z)dZ/dY+(1+dZ/dY)Z^2/(X+Z)=X
dZ/dY=(X-Z^2/(X+Z))/（2Zln（X+Z)+Z^2/(X+Z)）
所以dZ=(Y-Z^2/(X+Z))/（2Zln（X+Z)+Z^2/(X+Z)）dX+(X-Z^2/(X+Z))/（2Zln（X+Z)+Z^2/(X+Z)）dY