通常把“三角形任何两边的和大于第三边”叫做三角形的三边关系定理，它在三角形有关边的不等式的问题中最常用到。初学几何的同学普遍反映，三角形中不等式的证明是个难点，解（证）题时常常摸不着要领，感到无从下手。这在很大程度上与学好用好三角形三边关系有关，那么，怎 The trilateral relational theorem for triangles is usually called the triangle on any two sides and greater than the third side. It is most commonly used in the problem of triangle inequality. The students of the beginner geometry generally reflected that the proof of the inequality in the triangle is a difficult point. When solving (certificate) questions, they are often unable to grasp the essentials and feel that they cannot start. This is largely related to learning how to use the triangular trilateral relationship.