x2+y2-4x-6y+12=0
(x-2)^2+(y-3)^2=1
x^2+y^2为(x-2)^2+(y-3)^2=1上（x,y)到(0,0)的距离.则
(2,3)到(0,0)的距离=√13
x^2+y^2的最小值为：√13-1；最大值为:√13+1