2y^2>=0
2y^2=6x-3x^2>=0
3x(x-2)<0
0<=x<=2
y^2=(6x-3x^2)/2
x^2+y^2=-x^2/2+3x=-(1/2)(x-3)^2+9/2
0<=x<=2
所以x=0,最小=0
x=2,最大=4