配方:(x-2m)^2+(y-1)^2=4m^2-8m+8
(x-2m)^2+(y-1)^2=4(m-1)^2+4
面积最小,即半径最小,而r^2=4(m-1)^2+4
当m=1时,半径最小为2,此时面积最小.