y-yx+yx^2=1-xyx^2+(1-y)x+y-1=0当y不等于0时，△>=0(1-y)^2-4y(y-1)>=0(y-1)(y-1-4y)>=0(y-1)(3y+1)<=0所以 -1/3<=y<=1 且y不等于0
