平面几何的《四边形》一章内容丰富 ,非常重要。它是在三角形的基础上进一步学习的 ,与三角形知识关系非常密切。可以这样说 :四边形一章许多知识的展开、许多定理的证明、许多问题的解决 ,是建立在三角形的基础知识之上的。因此 ,《四边形》一章的学习 ,要十分注意的一个问题是 :学会转化 ,注意把四边形、多边形问题转化为三角形问题来解决 The “quadrilateral” chapter of plane geometry is rich in content and very important. It is further studied on the basis of triangles and is closely related to knowledge of triangles. It can be said that the development of a lot of knowledge in the quadrilateral chapter, the proof of many theorems, and the solution of many problems are based on the basic knowledge of triangles. Therefore, one issue to which we must pay attention in the study of the “Quadragon” chapter is to learn to transform and pay attention to translating quadrilateral and polygonal problems into triangular problems.