yx+2y-x+1=0;
真正的分离变量法!
(y-1)x+2y+1=0;
x=(-1-2y)/(y-1);
条件x≥-1
(-1-2y)/(y-1)≥-1
[-1-2y+y-1]/(y-1)≥0
(-y-2)/(y-1)≥0
(y+2)(y-1)≤0
因y-1是分母所以y=1舍
所以解得-2≤y