dy/【y（y-1）】 = dx
dy/(y-1) - dy/y = dx
d(y-1)/(y-1) - dy/y = 1·dx
ln/ （y-1）/y / = x + C ,代入x = 0 ,y = 1 ,得：
C = 0 ,解为 y = 1 + y·e^x