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数学 / 几何原本 / Proposition IX.34

第9卷命题 34 · 非二倍幂且半非奇数为兼偶

If a number neither be one of those which are continually doubled from a dyad, nor have its half odd, it is both eventimes even and even-times odd.

如果一个数既不是从2开始连续加倍得到的数之一,也没有其半数为奇数,那么它既是偶倍偶数又是奇倍偶数。

A A B B
fig-1

数 A 既不属于从 2 加倍数列,半也不为奇数;连续二分必到达奇数因子,故 A 既是偶倍偶数也是偶倍奇数。

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分步证明Step-by-step proof
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  1. For let the number A neither be one of those doubled from a dyad, nor have its half odd; I say that A is both even-times even and even-times odd. Now that A is even-times even is manifest; for it has not its half odd.

    设数A既不是从2开始连续加倍得到的数之一,也没有其半数为奇数。

  2. [VII. Def. 8] I say next that it is also even-times odd. For, if we bisect A, then bisect its half, and do this continually, we shall come upon some odd number which will measure A according to an even number.

    首先,A是偶倍偶数,因为它的半数不是奇数(根据定义VII.8)。

  3. For, if not, we shall come upon a dyad, and A will be among those which are doubled from a dyad: which is contrary to the hypothesis. Thus A is even-times odd.

    其次,若连续平分A及其一半,最终会得到一个奇数,该奇数以偶数倍量A;否则会得到2,从而A属于从2开始连续加倍的数,与假设矛盾。

  4. But it was also proved even-times even.

    因此A是奇倍偶数,且已证是偶倍偶数。