第二册《几何》课本指出了三角形三边之间的关系：三角形的两边之和大于第三边，两边之差小于第三边．这一关系在解题中有着广泛的应用．现举例说明．一、判断三条线段能否构成三角形例1下列各组线段中，一定能构成三角形的是（）（A）4，5，9；（B）7，10，2；（C）a＋2，2a＋ The second book “Geometry” pointed out the relationship between the three sides of the triangle: the sum of the sides of the triangle is greater than the third side, and the difference between the two sides is smaller than the third side. This relationship has a wide range of applications in problem solving. Here is an example. First, determine whether the three line segments can form a triangle. Example 1 In the following groups of line segments, it must be able to form a triangle: () (A) 4, 5, 9; (B) 7, 10, 2; (C) a + 2, 2a +