“补形法”是立体几何中较常用的基本方法之一,根据立体几何问题的条件和图形特征,将原题的图形补成一个常见的、规则的几何图形,利用补形后的图形的性质来解决原问题,往往会带来意想不到的方便.补形法不仅能大大地缩短从已知到未知的探求过程,使解题方法简洁、明快,而且还能逐步培养学生丰富的想象力,促进学生创造性思维的发展.1对称补形某些不规则几何体若存在对称性则可考虑用对称的方法进行补形,把它们放入一个 “Complementary method” is one of the more commonly used basic methods in three-dimensional geometry. According to the conditions and graphic features of the three-dimensional geometry problem, the original image is complemented into a common, regular geometric pattern, using the shape after the complement. The nature of the graph to solve the original problem will often bring unexpected convenience. The supplementation method can not only greatly shorten the search process from known to unknown, make the problem solving method concise and clear, but also can gradually cultivate students’ rich imagination. Strengthens the development of students’ creative thinking.1 Symmetrical complements If there is symmetry in some irregular geometry, consider symmetry for making up and putting them into a