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全国通用教材初中几何第二册第93页上有一个习题(28)介绍民间一种正五边形的近似画法,使用口诀“九五顶五九、八五分两边”。我们通过和木工师付的讨论,提出另一种更为精确的近似作法。因为木工一般不用圆规,主要用曲尺来划线,(曲尺是有刻度的直角尺)。那么如何使用一把曲尺可以较快地画一个比较精确的正五边形呢?我们认为下面的作法也可以介绍给学生,以培养学生学习的兴趣。
There is an exercise (28) on page 93 of Book 2 of the National General Textbook for Junior Middle School Geometry. This paper introduces the folk use of a regular pentagonal approximation, using the “Nine-Five-Nine Five-Nine, Eight-Fifty Two-Point” formula. We discussed with the carpenter to propose another more accurate approximation. Because woodworkers generally do not use compasses, they mainly use rulers to mark, (the ruler is a graduated square). So how can we use a ruler to quickly draw a more accurate regular pentagon? We think that the following practices can also be introduced to students to cultivate students’ interest in learning.