a+x>=0,a-x>=0
-a<=x<=a,所以a>=0
y>=0
y^2=2a+2根号(a^2-x^2)
y最小时,x^=a^2,y^2=2a=(根号2)^2=2
a=1
y最大时,x^2=0,y^2=2a+2|a|=4a=2^2=4
a=1
a=1