“知二补三”是培养学生读图能力的一种方法。主要包括两大类问题:一类是知道形体形状,根据两面投影补画第三视图;另一类是不知道形体的形状,利用两面投影并根据想象补画第三视图。笔者在长期的教学中发现学生对于这类问题的解决常常感到难以入手,没有思路。笔者在本文中根据自己的经验对于这两类问题的解答提出了一些解题思路与方法,并给出了实例加以演示。 “Know two make three ” is a way to develop students’ reading ability. Mainly include two major types of problems: one is to know the shape of the body, according to projection on both sides of the third view; the other is not aware of the shape of the body, the use of projection on both sides and according to the imagination make up the third view. In the long-term teaching, I found that students often find it hard to start with solutions to such problems without thinking. Based on my own experience, the author puts forward some solutions to these two kinds of problems and presents some examples to demonstrate them.