f(x)与y轴交于点(0,c)
f(x)=0 ==> x1 = 1+√(c+1),x2 = 1-√(c+1),所以圆的圆心一定落在直线x = 1上
令圆心(1,y)
所以:1+(y-c)^2 = c+1 + y^2
得y = (c-1)/2
所以圆方程:(x-1)^2 + [y-(c-1)/2]^2 = 1+(c+1)^2/4