两边对x求导得
1/[1+(y/x)^2]*(y/x)'=1/[ln(x^2+y^2) ] *[ln(x^2+y^2) ] '
1/[1+(y/x)^2]*(y'x-y)/x^2=1/[2ln(x^2+y^2) ] *1/(x^2+y^2) *(x^2+y^2) '
1/[1+(y/x)^2]*(y'x-y)/x^2=1/[2ln(x^2+y^2) ] *1/(x^2+y^2) *(2x+2yy')
1/[x^2+y^2]*(y'x-y)=1/[2ln(x^2+y^2) ] *1/(x^2+y^2) *(2x+2yy')
(y'x-y)=1/[2ln(x^2+y^2) ] *(2x+2yy')
解出来y'即可