2x^2-6x+y^2=0
y^2=6x-2x^2>=0
x^2-3x<=0
0<=x<=3
原式=x^2-2ax+y^2
=x^2-2ax+6x-2x^2
=-x^2+(6-2a)x
=-(x+(3-a))^2+(3-a)^2
當(dāng)3-a>0即a<3時(shí),x=3時(shí)原式最小,此時(shí)原式=3^2+(6-2a)*3=27-6a為最小值
當(dāng)3-a=0時(shí),即a=3時(shí),x=3時(shí)原式最小,此時(shí)原式=-x^2=-9
當(dāng)-1.5<3-a<0時(shí),即3當(dāng)3-a<=-1.5時(shí),即a>=4.5時(shí),x=0是原式最小,此時(shí)原式=0