首先配方，将式子变成（x-1）^2+(y-1)^2=1，因为0<=（x-1）^2<=1，0<=（y-1）^2<=1,所以可以得出1<=x<=2，1<=y<=2，然后-1<=x-2<=0，-3<=y-4<=-2所以y-4/x-2的取值范围0到1/2
