定义 1　对图形Ｇ ,若同一平面上存在点Ｏ ,使得将平面绕Ｏ旋转 1 80°时 ,Ｇ与自身完全重合 ,则Ｇ称为中心对称图形 ,Ｏ称为对称中心 ,上述旋转中重合的两点 ,称为对称点 .那么显然有 :(1 )封闭中心对称图形上对称点的连线 ,必过对称中心 ,且被其平分 ;(2 )封闭中心对称图形恰 Defining a pair of graph G, if there is a point O on the same plane, when the plane is rotated by 180° around O, G and itself completely coincide, then G is called center symmetrical figure, O is called symmetrical center, and the above rotation is coincident Two points are called symmetry points. Then there are obviously: (1) The line connecting the symmetrical points on the symmetrical figure of the closed center must pass through the center of symmetry and be divided equally; (2) The symmetrical figure of the closed center is exactly