ON THE THEOREM OF SHEN GUA
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Transcript of ON THE THEOREM OF SHEN GUA
ON THE THEOREM OF SHEN GUA Abstract The Chinese mathematician and philosopher Shen Gua (11thAD) has developed a simple and beautiful formula for the approximated calculation of the arc of a circle. The Chinese text has been translated into English by Wagner and by Martzloff. Since their mathematical explanations seem to me very complicated, I give here a simple and complete derivation of the theorem. It is based on the term “shift one place“ translated from the Chinese words “tui yi wei”. I am a physicist and I don’t know the Chinese language. I show, however that “shift one place” is symbolically equivalent with the Binomial Theorem which is well known in the Chinese and the European mathematics. Contents 1 ) The theorem of Shen Gua 2 ) Pythagoras 3 ) Babylon 4 ) Proof of Shen Gua 5 ) Archimedes 6 ) Pascal 7 ) Newton 8 ) Conclusion 9 ) Literature 1) The theorem of Shen Gua Figure 1 Definitions: h = sagitta r = radius b = half cord 𝛼 = angle of half chord Formula of Shen Gua: Arc (2𝛼)= 2b + ℎ!/r
2 ) THE THEOREM OF PYTHAGORAS Figure 2 Definitions 𝛼 = angle (p,a) 𝛽 = angle(q,b) Demonstrations: cos(𝛼) = p/a = a/c 𝑎! = pc (Ahmed) cos(𝛽) = q/b = b/c 𝑏! = 𝑞𝑐 Pythagoras1: 𝑎!+ 𝑏! = (p +q)c = 𝑐! (Euklides) Height: tg (𝛼) = h/p = q/h ℎ! = pq (Euklides) Pythagoras2: pq = (r + x) × (r -‐ x) = 𝑟! -‐ 𝑥! = ℎ! (Babylon) Pythagoras3: 𝑟! = ℎ! + 𝑥! Pythagoras4: 𝑎!/𝑐! + 𝑏!/𝑐! = 𝑐𝑜𝑠! (𝛼) + 𝑠𝑖𝑛!(𝛼) = 1 (Hipparchos) The above algebraic demonstrations are based on the Similarity equations of the rectangular triangles which are transformed into Metric equations
ha
r
xq
b
p = r + x c = p + q p
3) Babylon The Babylonians have developed a method for an approximated calculation of a square root: The formula is: D1 = 1/2 (a + D/a) a is equal to the root oft the next lower square of D. D1 is the approximation to the square root of D The equation can be inductively iterated: D2 = ½(D1 + D/D1) Example : D=6 a=2 D1 = 1/2 (2 + 6/2 ) = 2.5 True value = 2.44 . . . Figure 3 Geometric proof 𝐷 = a + x D = ( 𝑎 + 𝑥)! = 𝑎! + 2 a x + 𝑥! If 𝑥! is a small value , it can be neglected. We get therefore: x = D/2a – a/2 and D1 = a + x = 1/2 (a + D/a) If a is equal to 1 , we get D1 = 1/2 + D/2 Consider now the root of ( 𝑎! + y) where y is small: 𝑎! + 𝑦 = 𝑎!(1+ 𝑦/𝑎!) = a ∙ 1+ 𝑦/𝑎!) = a + y/2a
This is the equation that has been used by Shen Gua .
4 ) Proof of the theorem of Shen Gua Shen Gua 1: Calculate s(b,h) From Figure 1 it follows that 𝑠! = 𝑏! + ℎ! = 𝑏! (1+ ℎ!/𝑏!) Pythagoras s = b 1+ ℎ!/𝑏! ≅ b + ½ ℎ!/b Babylon (Shift theorem) Arc = 2b + ℎ! / b = 2b + ℎ!/r sin(𝛼) problem: if b < 1 then the rest arc gets greater solution: multiply the rest arc by sin(𝛼) Arc = 2b + ℎ!/r Theorem of Shen Gua note : the new arc is smaller since sin( 𝛼) is always ≤ 1 Shen Gua 2: Calculate b(h) and arc(r,h)
b = 𝑟! – 𝑎! = 𝑟! – (𝑟– ℎ)! Pythagoras Arc = 2b + ℎ! / r Shen Gua 3 Example with r=5 and h=2
Result: a = r–h=3 b = 5! – 3! = 4 arc = 8 + 0.8 = 8.8 note: this ist the well known Pythagorian triangle 3,4,5
5 ) Archimedes Archimedes (3.BC) has calculated the number 𝜋 with regular polygons that are inscribed in a circle. He used the following equations (see Figure 1): 𝑟! = 𝑎! + 𝑏! 𝑠! = 𝑏! + ℎ! if r = 1 then a =cos𝛼 ℎ! = (𝑟–𝑎)! and b=sin 𝛼 polygon side equation: 𝑠! = 𝑠𝑖𝑛!𝛼 + (1– cos𝛼)! = 𝑠𝑖𝑛!𝛼 + 𝑐𝑜𝑠!𝛼 + 1 – 2cos𝛼
= (2 sin𝛼/2)! = 2 (1-‐cos𝛼) 𝑠𝑖𝑛! (𝛼/2) = 1/2 (1–cos 𝛼)
sin (𝛼/2) = 1/2(1– 1– 𝑠𝑖𝑛!𝛼 )
The derivation of this equation uses 3 times the theorem of Pythagoras and one time the Binom theorem. The equation shows that the sin of 𝛼/2 can be calculated with two square roots from the sin of 𝛼 . An iterative procedure gives therefore the sin of 𝛼 = 360°/(𝑛) . Numeric values are shown in the following table. Arc1=n∙ sin (𝛼/2) Arc2=2n∙sin(𝛼/4) ShenGua=(n/2) ∙ (2 sin(𝛼/2)+(1– 𝑐𝑜𝑠!(𝛼/2) ) n gone 𝛼 Arc1 Arc2 Shen Gua Error 2 gone 180° 2 2.8 3 4.5 % 3 gone 120° 2.5 3.0 2.97 5.4% 4 gone 90° 2.8 3.06 3.0 4.5% 5 gone 72° 2.93 3.09 3.02 3.8% 6 gone 60° 3.0 3.105 3.05 2.9% 12 gone 30° 3.105 3.13 3.11 1% 96 gone 3.75° 3.14103 𝜋 = 3.14159
6 ) PASCAL Pascal (17.AD) is famous for the discovery of the Pascal Triangle, which is equivalent with the Binomial Theorem. There are three methods for the development of this theorem: 1 1 1 1 1 1 1 1 2 3 4 5 6 1 3 6 10 15 1 4 10 20 1 5 15 1 6 1 The triangle is constructed with the arithmetic sum of 2 successive numbers written below. Example: 3 + 6 = 10 . Pascal used his triangle for the development of the binomial series: 1+ 𝑥 ! = 1 + 6 x + 15 𝑥! + 20 𝑥! + 15 𝑥! + 6 𝑥! + 1 𝑥! The binomial coefficients of the series can also be determined by multiplication: 𝑛𝑘 = n!/ k!(n-‐k)! i.e. 62 = 6 5 4 3 2 1 / (2 1) ( 4 3 2 1) = 15 This is the second method. The binomial coefficients are generally used in the Probability theory. Originally the Pascal triangle has already been discovered by Omar Kayyam (11.AD), and Yang Hui (13.AD), Al Kashi (15.AD) They used a Shift and Add operation: (1+ 𝑥)! = 1 1 1 1 (1+ 𝑥)! = 1 2 1 1 2 1 (1+ 𝑥)! = 1 3 3 1 1 3 3 1 (1+ 𝑥)! = 1 4 6 4 1 1 4 6 4 1 (1+ 𝑥)! = 1 5 10 10 5 1
7 ) NEWTON One of the great discoveries of Newton (17.AD) was, that the definition of the binom coefficients could be extended to fractional exponents: For an exponent of 1/2 one gets 1+ 𝑥 = (1+ 𝑥)!/! = 1 + 1/2 x – 1/8 𝑥! + . . . The series extends to the infinite. It is convergent for x ≤ 1. It is a shift N place operation. It can be proved with the Taylor series of f(x): y = f(x) = f(0) + 𝑓(!)(0) ∙ x + 𝑓(!)(0)/2! ∙ 𝑥! + . . . 𝑓(!)(0)/n! ∙ 𝑥! where 𝑓(!) denotes the derivative of the order n. The connection with the Shen problem is, that Shen uses only the shift one term. Since the second term is negative, the Shen approximation is too high. 8 ) Conclusion My comment to the paper of Wagner is: He writes, that we have to leave out the words “shift one place“ for an understanding of the text and he thinks that these words have been added by a later editor of the Shen Gua text. My opinion is: The term “shift one place” is the key to the understanding of the Shen Gua formula. If we leave out these words, actually we can’t understand the Shen Gua formula. Moreover, if we leave out shift N operations, we can forget a great part in many mathematical books. My comment to the book of Martzloff is: In his text he writes, that Shen Gua did not explain his equation. This is not true. The term “shift one place“ is a short and very precise explanation of Shen Gua’s formula. 9 ) Literature 9.1 Donald B.Wagner : Shen Gua and an ignorant editor on the length of an arc in: Google: <http:/donwagner.dk/Shen-‐Gua-‐arc.htm> 9.2 Jean – Claude Martzloff: A History of Chinese Mathematics 9.3 Karine Chemla et Guo Shuchun: Les neuves chapitres 9.4 B. L. van der Waerden: Erwachende Wissenschaft 9.5 Carl B. Boyer: A History of Mathematics