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Note that is the traditional Chinese character for , and the mentioned below all refer to .

勾股幂合以成弦幂

variant:

  1. 合勾股幂以成弦幂

  2. 以勾股幂合成弦幂

According to ancient grammar, there are two variations of this sentence, and I prefer the former.

  • represents the short right-angled side, like a in the formula.
  • represents the long right-angled side, like b in the formula.
  • means the product of two numbers multiplied together, here denoting self-multiplication, like the concept of square in modern math.
    • 勾股幂 can be understood as the omitted expression of 勾幂和股幂.
  • means 结合, represents combine. It can be simply understood here as the sum of 勾幂 and 股幂.
  • 以成 means 用来形成, represents to become/form in English..
    • means 用来, represents the word to in English.
    • means 变成 or 形成, represents become or form in English.
  • represents the hypotenuse of a right triangle, like c in the formula.
  • as above.

Origin of

about (towel) :

巾,佩巾也。——《说文解字》

is a pictographic character. In the oracle bone script, its glyphs resemble a piece of fabric hanging down at both ends, indicating a textile for wiping, covering, wrapping, wearing, etc., much like modern towel.


about (power) :

大巾谓之幂。——《小尔雅·广诂》

is a phonetic word, and its sound side is , indicating that the word and have similarities in pronunciation. Its shape is , indicating that the word is related to the word .

According to the structure of , it can be inferred that its original meaning is a towel used to cover things. It is then derived to mean to cover(覆盖,遮掩), and from that to mean area(面积).


about :

“冖,覆也,从一下垂也。” ——《说文解字》

Cover something with a square piece of cloth and let the four corners hang down to form the shape of an .


Extending this meaning, anything that is square can also be called a . By further extension, the area of a rectangle(矩形)1 or the product of two numbers2(especially the result of multiplying a number by itself [3]) is also called a power. This promotion began with Liu Hui(an ancient mathematician called 刘徽 in China).

"the product of two numbers" means 两数之积 in Chinese. "multiplying a number by itself" means 自乘(一个数与自身相乘) in Chinese.

1

方田术曰:广纵步数相乘得积步。此积谓田幂。凡广纵相乘谓之幂。——《九章算术·方田》

译文 长方形田面积法则:宽与长的步数相乘得积步。(刘徽注:这个积称为田的幂。凡是长宽相乘就称为幂。)

The area rule of a rectangular field: multiply the number of steps of width and length to get the product of steps. (Liu Hui note: This product is called the (power) of the field. Where length and width are multiplied together they are called .)

(step) : In ancient times, each of the two feet crossed once called step, now refers to the distance between the two feet when walking.

2

又,勾自乘,以减弦自乘。其余,开方除之,即股。勾、股幂合以成弦幂,令去其一,则余在者皆可得而知之。——《九章算术·勾股》

译文:计算勾的自乘数,然后用弦的自乘数减去它,把余数作开方(根号)运算,结果就是股。

Calculate the self-multiplier of the , then subtract it from the self-multiplier of the , take the square root of the remainder, and the result is .

[3]

自乘之数曰幂。——《几何原本》by 徐光启, 利马窦


Now go back to the Pythagorean theorem(毕达哥拉斯定理,勾股定理).

Typically, the mathematical expression we use to describe the Pythagorean Theorem is a^2 + b^2 = c^2.

At the same time, we usually think of the hypotenuse of a right triangle as c, the short right side as a, and the long right side as b.

Therefore, a is shorter than b, and b is shorter than c.(a <= b, b < c)

Description of Pythagorean theorem in ancient Chinese mathematics

短面曰勾,长面曰股,相与结角曰弦。勾短其股,股短其弦。——《九章算术·勾股》

译文: 在直角三角形中,短的边称为勾,长的边称为股,与它们分别形成直角的边为弦。勾的长度比股短,股的长度比弦短。

In a right triangle, the short side is called , the long side is called , and the side forming a right angle with them is .

is shorter than , and is shorter than .(勾 <= 股股 < 弦

enter image description here


What does 朱幂 and 黄幂 mean in this ancient illustration of the Pythagorean Theorem?

about 朱幂 and 黄幂 :

here means yellow.

here means square, can be replaced by the word .

enter image description here

As can be seen from the picture:

朱幂 = 1/2 × (勾 × 股) = 1/2 × ab

黄幂 = (勾 - 股) × (勾 - 股) = (b - a)^2

弦幂 = 弦 × 弦 = c^2

The first kind of argument:

Four congruent right triangles form a square(弦幂) with side length c, and in the middle of the figure there is a small square(勾股差幂) with side length b – a.

because:

弦幂 = 4 × 朱幂 + 黄幂

thus:

c^2 = 1/2 × ab × 4 + (b - a)^2

c^2 = 2ab + a^2 + b^2 - 2ab

finally:

c^2 = a^2 + b^2

The second argument:

Then splice four identical congruent right triangles(朱幂) outside the square(弦幂) with side length c, and you will have a square with side length a + b, which can also prove the Pythagorean theorem.

because:

(勾股差幂 + 4 × 黄幂 ) + 4 × 黄幂 = (a + b)^2

弦幂 + 4 × 黄幂 = (a + b)^2

thus:

c^2 + 4 × (1/2 ab) = (a + b)^2

c^2 + 2 × ab = a^2 + b^2 + 2 × ab

finally:

c^2 = a^2 + b^2

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