《測圓海鏡》勾股形“月山泛”﹝13﹞之五和五較說
《測圓海鏡》勾股形“月山泛”﹝13﹞之五和五較說
上傳書齋名:瀟湘館112 Xiāo XiāngGuǎn 112
何世強 Ho Sai Keung
提要:《測圓海鏡》乃金‧李冶所撰,其書之“圓城圖式”含十四勾股形,連同原有之大勾股形共十五勾股形。此等勾股形三邊形成一系列之恆等式,本文主要談及第13勾股形月山泛相關之五和五較等式。
關鍵詞:太虛弦、太虛股、太虛勾、月山泛
《測圓海鏡》乃金‧李冶所撰,書成於 1248 年,時為南宋淳祐八年。該書卷一“圓城圖式”主要討論與十五勾股形相關之等式,本文介紹其部分等式並作出証明。
本文所引用之勾股式源自“圓城圖式”之十五勾股形,a1、b1、c1 乃最大勾股形天地乾之勾、股及弦長。故 a1、b1、c1 又稱為大勾﹝地乾﹞、大股﹝天乾﹞及大弦﹝天地﹞。
《測圓海鏡》之〈諸雜名目〉篇涉及一系列之勾股恆等式,所有恆等式皆與十五勾股形有關。十五勾股形中最大者為天地乾,其三邊勾股弦分別以a1、b1、c1 表之,其餘十四勾股形三邊勾股弦則分別以 ai、bi、ci 表之,其中 1 < i ≦ 15。但 ai、bi、ci 均可以 a1、b1、c1 表之,此乃《測圓海鏡》之精髓。注意勾股定理成立,即 ai2 + bi2 = ci2。
有關以 a1、b1、c1 表 ai、bi、ci 之式可參閱筆者另文〈《測圓海鏡》“圓城圖式”之十二勾股弦算法〉。
以下左為“圓城圖式”右為“圓城圖式十五句股形圖”。
本文談及之勾股形乃“月山泛”,一作“月山水”﹝又稱為“太虛”,或簡稱為“虛”﹞,筆者有文談論以“月山水”為佳。
以下兩圖帶藍色之二勾股形即為“月山泛”,月山泛之斜邊“山地”是為太虛弦,其直角為 13,以 13 之位置為 “泛”,其勾股與弦分別為 a13﹝月山﹞、b13﹝月泛﹞ 與 c13﹝山泛﹞。
注意以下之“月山水”名稱:
注意圓徑為 a1 + b1 – c1,見上圖之東南西北。
以下為月山泛勾股形之三事﹝三事,三邊之長也﹞:
太虛勾:a13 =
(c1 – b1)(c1 – a1)。
太虛股:b13 =
(c1 – b1)(c1 – a1)。
太虛弦:c13 =
(c1 – b1)(c1 – a1)。
山地艮勾股形之三事和或較亦可以以 a1、b1、c1 表之。
若勾股形之弦 = c,勾 = a,股 = b,則以下為五和五較:
(1) 勾股和:a + b
(2) 勾股較:b – a
(3) 勾弦和:a + c
(4) 勾弦較:c – a
(5) 股弦和:b + c
(6) 股弦較:c – b
(7) 弦較和:c + (b – a) ﹝較指勾股較,和指弦與勾股較之和﹞
(8) 弦較較:c – (b – a) ﹝第一較字指勾股較,第二較字指弦與勾股較之較﹞
(9) 弦和和:(a + b) + c ﹝第一和字指勾股和,第二和字指弦與勾股和之和。又稱為三事和﹞
(10) 弦和較:(a + b) – c ﹝第一和字指勾股和,第二較字指弦與勾股和之較。又稱為三事較﹞
以下為與太虛弦﹝勾股形山地艮 (13)﹞有關五和五較之等式:
太虛勾股和即圓徑內減虛弦。又為虛弦虛黃方共。又為皇極弦內去明股
勾共。其差則大差勾內減個小差股也。勾弦共即小差股也。其較則虛股內減個小黃方也。股弦共即大差勾。其較則虛勾內減個小黃方也。弦較和為大差弦上弦和較。又黃長弦上勾弦較。又為兩個明勾。其較小差弦上黃方面也。三事和即大黃方。其較則為兩個明弦上股弦較。又為
弦上兩個勾弦較。又為明弦上小差與
弦上大差共也。
以下為各條目之証明:
太虛勾股和即圓徑內減虛弦。
太虛勾股和=b13 + a13 =
(c1 – b1)(c1 – a1) +
(c1 – b1)(c1 – a1)]
= (c1 – b1)(c1 – a1)[
+
]
=
(c1 – b1)(c1 – a1)(b1 + a1)。
已知圓徑為 a1 + b1 – c1,虛弦 = c13 =
(c1 – b1)(c1 – a1)。
圓徑內減虛弦 = (a1 + b1 – c1) –
(c1 – b1)(c1 – a1)
= (a1 + b1 – c1) –
(a1 + b1 – c1)2
= (a1 + b1 – c1)[1 –
(a1 + b1 – c1)]
=
(a1 + b1 – c1)(2a1b1– c1a1 – c1b1 + c12)
=
(a1 + b1 – c1)(2a1b1– c1a1 – c1b1 + a12 + b12)
=
(a1 + b1 – c1)[(a1 + b1)2 – c1(a1 + b1)]
=
(a1 + b1 – c1)(a1 + b1)[(a1 + b1) – c1]
=
(a1 + b1 – c1)2(a1 + b1)
=
(c1 – b1)(c1 – a1)(b1 + a1)。
注意等式 (c1 – b1)(c1 – a1) =
(a1 + b1 – c1)2。
所以太虛勾股和 = 圓徑內減虛弦。
又為虛弦虛黃方共。
已知虛黃方 = a13 + b13 – c13
=
(c1 – b1)(c1 – a1) +
(c1 – b1)(c1 – a1) –
(c1 – b1)(c1 – a1) 。
﹝黃方定義可參閱筆者另文。﹞
虛弦虛黃方共
=
(c1 – b1)(c1 – a1) +
(c1 – b1)(c1 – a1) +
(c1 – b1)(c1 – a1) –
(c1 – b1)(c1 – a1)
=
(c1 – b1)(c1 – a1) +
(c1 – b1)(c1 – a1)
= (c1 – b1)(c1 – a1)[
+
]
=
(c1 – b1)(c1 – a1)(b1 + a1)。
很明顯太虛勾股和= 虛弦 + 虛黃方。
又為皇極弦內去明股
勾共。
已知皇極弦﹝日川﹞:c12 =
(a1 + b1 – c1) 。
明股:b14 =
=
(c1 – a1)(b1 – c1 + a1)。
勾:a15 =
(c1 – b1)(a1 – c1 + b1)。
皇極弦內去明股
勾
= c12 – (b14 + a15)= c12 – b14 – a15
=
(a1 + b1 – c1) –
(c1 – a1)(b1 – c1 + a1) –
(c1 – b1)(a1 – c1 + b1)
=
(a1 + b1 – c1)[
–
(c1 – a1) –
(c1 – b1)]
=
(a1 + b1 – c1)(c12 –b1c1 + b1a1 – a1c1 + a1b1)
=
(a1 + b1 – c1)(a12+ b12 – b1c1 + 2b1a1 – a1c1)
=
(c1 – b1)(c1 – a1)(b1 + a1)﹝步驟同前﹞。
所以太虛勾股和 = 皇極弦內去明股
勾共。
其差則大差勾內減个小差股也。
“其差”指太虛勾股較。
太虛勾股較 = b13 – a13 =
(c1 – b1)(c1 – a1) –
(c1 – b1)(c1 – a1)]
= (c1 – b1)(c1 – a1)[
–
]
=
(c1 – b1)(c1 – a1)(b1 – a1)。
已知坤月勾﹝又稱大差勾﹞:a10 =
=
(c1 – a1) 。
山艮股﹝又稱小差股﹞:b11 =
=
(c1 – b1) 。
大差勾內減個小差股= a10 – b11
a10 – b11 =
(c1 – a1) –
(c1 – b1)
=
[a12(c1 – a1) – b12(c1 – b1)]
=
[a12c1 – a13 –b12c1 + b13]
=
[(b1 – a1)(a12+ b1a1 + b12)– c1(b1 – a1)(b1 + a1)
=
[(b1 – a1)(c12+ b1a1) – (b1 – a1)(c1b1 + c1a1)]
=
(b1 – a1)(c12+ b1a1 – c1b1 – c1a1)
=
(b1 – a1)[c1(c1 – b1) – a1(c1 – b1)]
=
(b1 – a1)(c1 – b1)(c1 – a1)。
所以太虛勾股較 = 大差勾內減個小差股。
勾弦共即小差股也。
太虛勾弦共=c13 + a13 =
(c1 – b1)(c1 – a1) +
(c1 – b1)(c1 – a1)
=
(c1 – b1)(c1 – a1)[
+ 1]
=
(c1 – b1)(c1 – a1)(c1 + a1)
=
(c1 – b1)(c12 – a12)
=
(c1 – b1)b12
=
(c1 – b1)
= 小差股﹝見前式﹞。
所以太虛勾弦共 = 小差股。
其較則虛股內減個小黃方也。
“其較”指太虛勾弦較。“小黃方”指勾股形“月山泛”之黃方,即
a13 + b13 – c13。
太虛勾弦較=c13 – a13=
(c1 – b1)(c1 – a1) –
(c1 – b1)(c1 – a1)
=
(c1 – b1)(c1 – a1)[
– 1]
=
(c1 – b1)(c1 – a1)(c1 – a1)
=
(c1 – b1)(c1 – a1)2。
虛股內減個小黃方= b13 – (a13 + b13– c13)
= c13 – a13
= 太虛勾弦較
=
(c1 – b1)(c1 – a1)2﹝見前式﹞。
股弦共即大差勾。
太虛股弦共= c13 + b13 =
(c1 – b1)(c1 – a1) +
(c1 – b1)(c1 – a1)
=
(c1 – b1)(c1 – a1)[
+ 1]
=
(c1 – a1)(c1 – b1)(c1 + b1)
=
(c1 – a1)(c12 – b12)
=
(c1 – a1)a12
=
(c1 – a1) 。
已知坤月勾﹝又稱大差勾﹞:a10 =
=
(c1 – a1) 。
所以太虛股弦共= 大差勾。
其較則虛勾內減個小黃方也。
“其較”指太虛股弦較。
太虛股弦較= c13 – b13 =
(c1 – b1)(c1 – a1) –
(c1 – b1)(c1 – a1)
=
(c1 – b1)(c1 – a1)[
– 1]
=
(c1 – a1)(c1 – b1)(c1 – b1)
=
(c1 – a1)(c1 – b1)2。
虛勾內減個小黃方 = a13 – (a13 + b13 – c13)= c13 – b13
= 太虛股弦較。
弦較和為大差弦上弦和較。
太虛弦較和=c13 + (b13 – a13) = c13+ b13 – a13。
c13 + b13 – a13
=
(c1 – b1)(c1 – a1) +
(c1 – b1)(c1 – a1) –
(c1 – b1)(c1 – a1)
= (c1 – b1)(c1 – a1)[
+
–
]
=
(c1 – b1)(c1 – a1)(c1 + b1 – a1)
=
(c1 – a1)(c12 – b12 –a1c1 + a1b1)
=
(c1 – a1)(a12 – a1c1 + a1b1)
=
(c1 – a1)(b1 + a1 – c1) 。
大差上三事較即弦和較 = (b10 + a10) – c10 = b10 + a10 – c10。
b10 + a10 – c10 =(c1 – a1) +
(c1 – a1) –
(c1 – a1)
= (c1– a1)[ 1 +
–
]
=
(c1 – a1)(b1 + a1 – c1) 。
所以太虛弦較和 = 大差弦上弦和較。
又黃長弦上勾弦較。
黃長弦上勾弦較 = c5 – a5=
(a1 + b1 – c1) –
(a1 + b1 – c1)
=
(a1 + b1 – c1)(c1 – a1)
=
[b1 –(c1 – a1)](c1 – a1)
=
(b1c1 – b1a1– c12 – a12+ 2c1a1)
=
(c1 – a1)(b1 – c1 + a1)。
所以太虛弦較和 = 黃長弦上勾弦較。
又為兩個明勾。
又已知明勾 = a14=
(c1 – a1)(b1 – c1 + a1),
兩個明勾= 2 ×
(c1 – a1)(b1 – c1 + a1)
=
(c1 – a1)(b1 – c1 + a1)。
所以太虛弦較和 = 兩個明勾。
其較小差弦上黃方面也。
“其較”指太虛弦較較。
太虛弦較較 = c13 –(b13 – a13) = c13 – b13+ a13。
c13 – b13 + a13
=
(c1 – b1)(c1 – a1) –
(c1 – b1)(c1 – a1) +
(c1 – b1)(c1 – a1)
=
(c1 – b1)(c1 – a1)(c1 – b1 + a1)
=
(c1 – a1)(c1 – b1)(c1 – b1 + a1)
=
(a1 + b1 – c1)2(c1 – b1 + a1)
=
(a1 + b1 – c1)[a1 –(c1 – b1)]( a1 + c1 – b1)
=
(a1 + b1 – c1)[a12 – (c1 – b1)2]
=
(a1 + b1 – c1)[a12 – c12 –b12 + 2b1c1]
=
(a1 + b1 – c1)[ –2b12 + 2b1c1]
=
(a1 + b1 – c1)(c1 – b1)。
小差弦上黃方面即弦和較。
小差弦上三事較= 小差弦上弦和較 =(a11 + b11) – c11 = a11+ b11 – c11
= –
(c1 – b1) +
(c1 – b1) + (c1 – b1)
=
(c1 – b1)( – c1 + b1 + a1) 。
所以太虛弦較較 = 小差弦上黃方面。
三事和即大黃方。
太虛弦三事和即弦和和 = c13 + b13 + a13。
c13 + b13 + a13
=
(c1 – b1)(c1 – a1) +
(c1 – b1)(c1 – a1) +
(c1 – b1)(c1 – a1)
=
(c1 – b1)(c1 – a1)(c1 + b1 + a1)
=
(a1 + b1 – c1)2(c1 + b1 + a1)
=
(a1 + b1 – c1)(a1 + b1 – c1)(c1 + b1 + a1)
=
(a1 + b1 – c1)[(a1 + b1)2– c12]
=
(a1 + b1 – c1)(a12 + b12 –c12 + 2a1b1)
=
(a1 + b1 – c1)×2a1b1
= a1 + b1 – c1
= 大黃方﹝即大圓徑﹞。
其較則為兩個明弦上股弦較。
“其較”指太虛弦三事較 = 弦和較 = b13 + a13 – c13。
– c13 + b13 + a13
= –
(c1 – b1)(c1 – a1) +
(c1 – b1)(c1 – a1) +
(c1 – b1)(c1 – a1)
=
(c1 – b1)(c1 – a1)(– c1 + b1 + a1)
=
(a1 + b1 – c1)2(– c1 + b1 + a1)
=
(a1 + b1 – c1)2(a1 + b1 – c1)
=
(c1 – a1)(c1 – b1)(a1 + b1 – c1) 。
注意 (c1 – b1)(c1 – a1) =
(a1 + b1 – c1)2。
明弦上股即明股:b14 =
=
(c1 – a1)(b1 – c1 + a1)。
明弦:c14 =
(c1 – a1)(b1 – c1 + a1)。
明弦上股弦較=c14 – b14
=
(c1 – a1)(b1 – c1 + a1) –
(c1 – a1)(b1 – c1 + a1)
=
(c1 – a1)(b1 – c1 + a1)[
– 1]
=
(c1 – a1)(c1 – b1)(a1 + b1 – c1)。
兩個明弦上股弦較 = 2×
(c1 – a1)(c1 – b1)(a1 + b1 – c1)
=
(c1 – a1)(c1 – b1)(a1 + b1 – c1) 。
所以太虛弦三事較 = 兩個明弦上股弦較。
又為
弦上兩個勾弦較。
弦上
勾:a15 =
(c1 – b1)(a1 – c1 + b1)。
弦上
弦:c15 =
(c1 – b1)(a1 – c1 + b1)。
弦上勾弦較 =
(c1 – b1)(a1 – c1 + b1) –
(c1 – b1)(a1 – c1 + b1)
=
(c1 – b1)(a1 – c1 + b1)(
– 1)
=
(c1 – a1)(c1 – b1)(a1 + b1 – c1)。
兩個勾弦較= 2 ×
(c1 – a1)(c1 – b1)(a1 + b1 – c1)
=
(c1 – a1)(c1 – b1)(a1 + b1 – c1)。
所以太虛弦三事較 =
弦上兩個勾弦較。
又為明弦上小差與
弦上大差共也。
因為明弦上小差= 明弦上股弦較
=
(c1 – a1)(c1 – b1)(a1 + b1 – c1)﹝見前﹞,
又因為
弦上大差 =
弦上勾弦較
=
(c1 – a1)(c1 – b1)(a1 + b1 – c1)。
以上二差共=2 ×
(c1 – a1)(c1 – b1)(a1 + b1 – c1)
=
(c1 – a1)(c1 – b1)(a1 + b1 – c1)。
所以太虛弦三事較 = 明弦上小差與
弦上大差共。
以下為《測圓海鏡》原文: