设 圆的标准方程为(X-a)^2+(y-b)^2=1....（1）
上式展开后得：X^2-2ax+a^2+y^2+2by+b^2=1
与原方程比较后可得：-2a=-1 -2b=1
即 a=1/2 b=-1/2 a^=1/4，b^2=1/4 代入（1）后得
(X-1/2)^2+(y+1/2)^2=1./2.
(X-1/2)的平方＋（y＋1/2）的平方=1/2
圆心(-1/2，1/2) r=√2/2
(x-1/2)^2+(y+1/2)^2=1/2
(X-1/2)^2+(Y+1/2)^2=1/2
