初中《几何》课本中给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.事实上,这个命题的逆命题也成立.这一点课本中未予指明,而在习题里却出现了需使用其逆命题的练习题.为使应用方便,不妨叙述为下面定理. The relationship between the three sides of a triangle is given in the “geometry” textbook of junior high school: 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. In fact, the converse of this proposition is also true. Not specified, but in the exercises there are exercises that require the use of its inverse proposition. For ease of use, it may be stated as the following theorem.