Note that 冪
is the traditional Chinese character for 幂
, and the 幂
mentioned below all refer to 冪
.
勾股幂合以成弦幂
variant:
合勾股幂以成弦幂
以勾股幂合成弦幂
According to ancient grammar, there are two variations of this sentence, and I prefer the former.
勾
represents the short right-angled side, likea
in the formula.股
represents the long right-angled side, likeb
in the formula.幂
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
.
合
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
, representsto become/form
in English..以
means用来
, represents the wordto
in English.成
means变成
or形成
, representsbecome
orform
in English.
弦
represents the hypotenuse of a right triangle, likec
in the formula.幂
as above.
Origin of 幂
about 巾
(towel) :
巾,佩巾也。——《说文解字》
巾
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 幂
(power) :
大巾谓之幂。——《小尔雅·广诂》
幂
is a phonetic word, and its sound side is 冥
, indicating that the word and 冥
have similarities in pronunciation. Its shape is 巾
, indicating that the word is related to the word 巾
.
According to the structure of 幂
, 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
:
“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 冖
.
Extending this meaning, anything that is square can also be called a 幂
. 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.
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
幂
(power) of the field. Where length and width are multiplied together they are called幂
.)
步
(step) : In ancient times, each of the two feet crossed once called step, now refers to the distance between the two feet when walking.
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
勾
, then subtract it from the self-multiplier of the弦
, take the square root of the remainder, and the result is股
.
[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
勾
, the long side is called股
, and the side forming a right angle with them is弦
.
勾
is shorter than 股
, and 股
is shorter than 弦
.(勾 <= 股
,股 < 弦
)
What does 朱幂 and 黄幂 mean in this ancient illustration of the Pythagorean Theorem?
about 朱幂
and 黄幂
:
黄
here means yellow.
方
here means square, can be replaced by the word 幂
.
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