x=2y+1设圆心纵坐标是a,则横坐标2a+1(x-2a-1)^2+(y-a)^2=r^2过原点和A(2,1),代入(0-2a-1)^2+(0-a)^2=r^2(2-2a-1)^2+(1-a)^2=r^2相减(2a+1)^2+a^2-(2a-1)^2-(a-1)^2=0[(2a+1)^2-(2a-1)^2]+[a^2-(a-1)^2]=0(2a+1+2a-1)...