《工程制图》教材中关于正等轴测图有“其三个轴向伸缩系数均相等,即p1=q1=r1=0.82,”这样的描述,初学者不知道这个系数怎么来的,包括对斜二侧的r=0.5也感到困惑。这篇文章利用空间解析几何知识给予推导,以期达到让相关学习者明白0.82和0.5的由来。 “Engineering drawing” teaching materials on the isometric “” The three axial expansion coefficient are equal, that is, p1 = q1 = r1 = 0.82, "This description, novice beginners do not know how to come from this coefficient, Including the two sides of the ramp r = 0.5 are also confused. This article uses spatial analytic geometry to derive it so that learners understand the origins of 0.82 and 0.5.