y²=-x²/2+x
y²>=0
則-x²/2+x>=0
x²-2x=x(x-2)<=0
0<=x<=2
x²+y²
=x²-x²/2+x
=x²/2+x
=(1/2)(x²+2x+1-1)
=(1/2)(x+1)²-1/2
0<=x<=2
在對稱軸x=-1右邊,增函數(shù)
x=0,x²/2+x=0
x=2,x²/2+x=4
所以0<=x²+y²<=4