(xy
(xy-x)dx+(xy+x-y-1)dy=0求积分因子 化方程为 x(y-1)dx+(x-1)(y+1)dy=0 两边同乘以1/（x-1）(y+1) 得xdx/(x-1)+(y+1)dy/(y-1)=0 所求的积分因子为 u=1/(x-1)(y+1). 通积分是 （x-1）[(y-1)^2]e^(x+y)=c