初级中学课本《几何》对三角形的定义是: 由三条线段首尾顺次连结所组成的图形叫做三角形。如右图,线段AB、BC、CA首尾顺次连结而成,根据上述定义,这一图形也是三角形。且有AB=AC+BC,这显然与定理“三角形任何两边之和大于第三边”相矛盾。也与人们的习惯相悖。因此,建议将三角形的定义改为: 由三条不在同一直线上的线段首尾顺次 The definition of the triangle in the junior high school textbook “Geometry” is: A figure composed of three line segments connected sequentially from beginning to end is called a triangle. As shown in the figure on the right, the line segments AB, BC, and CA are connected sequentially from the beginning and the end. According to the above definition, this figure is also a triangle. And there is AB=AC+BC, which is obviously contradictory to the theorem that the sum of any two sides of the triangle is greater than the third side. It is also contrary to people’s habits. Therefore, it is recommended that the definition of the triangle should be changed to: The sequence of three lines that are not on the same line