现行的初中平面几何课本对“圆”这一概念未直接给以定义,而是这样描述的:“当射线OA绕着它的端点O旋转一周时,射线上的一点(例如A)就画出一条线,这条线叫做圆。”对扇形和弓形是这样定义的:“一条弧和过这弧的端点的两条半径所组成的圆形,叫做扇形。”“一条弧和这弧所对的弦所组成的圆形,叫做弓形。”后来,把多边形定义为“封闭折线”;三角形被当作多边形的一种,即 The current junior high school plane geometry textbook does not directly define the concept of “circle” but describes it as follows: “When a ray OA rotates around its end point O, a point on the ray (for example, A) is drawn. A line, this line is called a circle.” For fans and bows, this is defined as: “The circle formed by an arc and the two radii passing through the ends of the arc is called a fan.” “An arc and the arc The circle formed by the strings is called a bow. Later, the polygon was defined as a ”closed polyline"; the triangle was considered as a kind of polygon, ie