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数学 / 几何原本 / Proposition VII.39

第7卷命题 39 · 求最小公倍数数

To find the number which is the least that will have given parts.

求一个最小的数,使其含有给定的几个部分。

A B C D E F G H
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分步证明Step-by-step proof
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  1. Let A, B, C be the given parts; thus it is required to find the number which is the least that will have the parts A, B, C. Let D, E, F be numbers called by the same name as the parts A, B, C, and let G, the least number measured by D, E, F, be taken. [VII. 36] Therefore G has parts called by the same name as D, E, F.

    设A、B、C为给定的部分,取与各部分同名的数D、E、F。

  2. [VII. 37] But A, B, C are parts called by the same name as D, E, F; therefore G has the parts A, B, C. I say next that it is also the least number that has.

    取被D、E、F量尽的最小数G(根据VII.36)。

  3. For, if not, there will be some number less than G which will have the parts A, B, C. Let it be H. Since H has the parts A, B, C, therefore H will be measured by numbers called by the same name as the parts A, B, C.

    则G含有与D、E、F同名的部分(根据VII.37),即含有A、B、C。

  4. [VII. 38] But D, E, F are numbers called by the same name as the parts A, B, C; therefore H is measured by D, E, F. And it is less than G : which is impossible.

    假设存在更小的数H也含有A、B、C,则H被D、E、F量尽(根据VII.38),但H小于G,矛盾。故G为最小。