f(x,y)=e^(xy)lnf(x,y)=xyf'x(x,y)/f(x,y)=y+xy'f'x(x,y)=(y+xy')e^(xy)
将xy看为整体，复合函数e^u的导数e^u*u'，所以求e^xy(xy)'，结果是e^xy*(y+xy')
