圆方程化为(x-m)²+(y-1)²=m²-4m+5 S=πr²r²=m²-4m+5=(m-2)²+1 所以当m=2时面积最小为π
x^2+y^2-2mx-2y+4m-4=0(x-m)^2+(y-1)^2= m^2-4m+5r^2 = m^2-4m+5 = (m-2)^2 +1min r^2 at m=2m=2, 圆C的面积最小
