配方：
x²+y²-2kx+k²-1=0 (x-k)²+y²=1
圆心坐标：x=k y=0
x²+y²-2(k+1)y+k²+2k=0 x²+y²-2(k+1)y+(k+1)²=1 x²+[y-(k+1)]²=1
圆心坐标：x=0 y=k+1
圆心距离=√[(k-0)²+(0-k-1)²]=√(2k²+2k+1)=√[2(k+1/2)²+1/2]
当k=-1/2时,两圆心有最小距离√2/2
√2/2