du=[（x+ay）dx+4ydy]/(x+2y)
u'y=4y/(x+2y)=(4y+2x-2x)/(x+2y)=2-2x/(x+2y)
对y积分：u=2y-xln(x+2y)+g(x)
u'x=-ln(x+2y)-x/(x+2y)+g'(x)
题目不对题目哪里错了分母是否是(x+2y)^2