1/2*ln(x^2+y^2)=arctany/x两边对x求导,得
1/2*1/(x^2+y^2)*(2x+2y*y')=1/[1+(y/x)^2]*(y'*x-y)/x^2
化简得
y'=(x+y)/(x-y)
则dy=(x+y)/(x-y)*dx