《測圓海鏡》之太虛弦﹝7末篇﹞等式說 上傳書齋名:瀟湘館112 Xiāo Xiāng Guǎn 112 何世強 Ho Sai Keung 提要:《測圓海鏡》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,本文主要談及各勾股形與太虛弦相關之等式,太虛弦在勾股形月山泛。 關鍵詞:太虛弦、弦、虛黃、極雙差、傍差 《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》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 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”。本文主要談及月山太虛弦,見下圖藍色三角形之斜邊。 筆者已有文談及太虛弦之等式名為〈《測圓海鏡》圓城圖之太虛弦﹝6﹞等式說〉,本文乃其延續。 上圖第 13 點為“泛”﹝一作“水”﹞,第 7點為“朱”,第 12 點為“心”,第 8 點為“青”。 注意圓徑為 a1 + b1 – c1,見上圖之東南西北圓。 以下為與太虛弦﹝在勾股形月山泛 13﹞有關之等式: 太虛弦: 內減股即明勾。內加弦即極勾。內減弦為明勾內少個小差。加入和即兩個虛弦內少個平差也。內減和即平差也。加入明二和共即極和內少個虛黃也。若減於明二和共即明股勾共也。減於髙弦即明弦。減於平弦即弦。加於角差即二明勾一極差也。減於角差即一極差二股較也。得傍差即明股勾共。內減傍差即太虛三事和內去了極雙差也﹝按雙差係勾弦差、股弦差﹞。內加虛差即二明勾。內減虛差即二股。內加虛黃方即虛和。內減虛黃方即太虛大小差併也。 注意月山太虛弦﹝簡稱太虛弦﹞:c13 =(c1 – b1)(c1 – a1),見上圖。 注意等式 (c1 – b1)(c1 – a1) = (a1 + b1 – c1)2,所以 太虛弦 c13 = (c1 – b1)(c1 – a1) = (a1 – c1 + b1)2。 本文乃“諸弦”篇之末篇。 以下為各條目之証明: 內減股即明勾。 已知山東股﹝又稱股﹞:b15 =(c1 – b1)(a1 – c1 + b1)。 太虛弦內減股,即 c13 – b15 : c13 – b15 = (c1 – b1)(c1 – a1) –(c1 – b1)(a1 – c1 + b1) = (a1 – c1 + b1)2 –(c1 – b1)(a1 – c1 + b1) = (a1 – c1 + b1)[(a1 – c1 + b1) – (c1 – b1)] = (a1 – c1 + b1)(c1b1 + c1a1 – c12 – c1b1 + b12) = (a1 – c1 + b1)(c1a1 – c12 + b12) = (a1 – c1 + b1)(c1a1 – a12 – b12 + b12) = (a1 – c1 + b1)(c1a1 – a12) = (c1 – a1)(b1 – c1 + a1)。 已知南月勾﹝又稱明勾﹞:a14 = (c1 – a1)(b1 – c1 + a1)。 比較兩式,可知太虛弦內減股 = 明勾。 內加弦即極勾。 山川弦﹝簡稱弦﹞:c15 =(c1 – b1)(a1 – c1 + b1)。 太虛弦內加弦,即: (c1 – b1)(c1 – a1) + (c1 – b1)(a1 – c1 + b1) = (a1 – c1 + b1)2 + (c1 – b1)(a1 – c1 + b1) = (a1 – c1 + b1)(a1 – c1 + b1 + c1 – b1) = (a1 – c1 + b1) × a1 = (a1 + b1 – c1)。 已知皇極勾﹝川心﹞:a12 = = (a1 + b1 – c1)。 比較兩式,可知內加弦 = 極勾。 減於弦為明勾內少個小差。 已知弦:c15 = (c1 – b1)(a1 – c1 + b1)。 太虛弦減於弦,即: –(c1 – b1)(a1 – c1 + b1) + (c1 – b1)(c1 – a1) = –(c1 – b1)(a1 – c1 + b1) + (a1 – c1 + b1)2 =(a1 – c1 + b1)[ – (c1 – b1) + (a1 – c1 + b1)] =(a1 – c1 + b1)( – c1 + b1 + a1 – c1 + b1) =(a1 – c1 + b1)( –2c1 + 2b1 + a1)。 已知明勾:a14 =(c1 – a1)(b1 – c1 + a1)。 “小差”即弦上股弦較 = c15 – b15 。 c15 – b15= (c1 – b1)(a1 – c1 + b1) –(c1 – b1)(a1 – c1 + b1) = (c1 – b1)(a1 – c1 + b1)[– 1] = (c1 – b1)(a1 – c1 + b1)(c1 – b1) = (c1 – b1)2(a1 – c1 + b1) 。 “明勾內少個小差”即: (c1 – a1)(b1 – c1 + a1) –(c1 – b1)2(a1 – c1 + b1) =(b1 – c1 + a1)[(c1 – a1) –(c1 – b1)2] = (b1 – c1 + a1)(c1a1 – a12 – c12– b12 + 2c1b1) = (b1 – c1 + a1)(c1a1 – 2c12 + 2c1b1) =(a1 – c1 + b1)( –2c1 + 2b1 + a1)。 比較兩式,可知太虛弦減於弦 = 明勾內少個小差。 加入和即兩個虛弦內少個平差也。 “和”即弦上勾股和 = b15 +a15 。 b15 + a15 = (c1 – b1)(a1 – c1 + b1) +(c1 – b1)(a1 – c1 + b1) = (c1 – b1)(a1 – c1 + b1)(+) = (c1 – b1)(a1 – c1 + b1)(b1 + a1) 。 太虛弦加入和,即: (c1 – b1)(c1 – a1) + (c1 – b1)(a1 – c1 + b1)(b1 + a1) = (a1 – c1 + b1)2 + (c1 – b1)(a1 – c1 + b1)(b1 + a1) = (a1 – c1 + b1)[c1(a1 – c1 + b1) + (c1 – b1)(b1 + a1)] = (a1 – c1 + b1)[c1b1 + c1a1 – c12 + c1b1 + c1a1 – b12 – b1a1] = (a1 – c1 + b1)[c1b1 + c1a1 – c12 + c1b1 + c1a1 – b12 – b1a1] =(b1 + a1 – c1)[2c1b1 + 2c1a1 – c12 – b12 – b1a1]。 “平差”指平弦﹝在勾股形月川青 8 或川地夕 9﹞上勾股較。 平弦上勾股較 = b8– a8 = (a1 + b1 – c1) –(a1 + b1 – c1) = (a1 + b1 – c1)(1 –) =(a1 + b1 – c1)(b1 – a1) =(b12 –a12 – c1b1 + c1a1) =[(b1 – a1)(b1 + a1) – c1(b1 – a1)] =(b1 – a1) (b1 + a1 – c1)。 兩個虛弦﹝在勾股形月山泛 13﹞內少個平差﹝兩個即乘以 2﹞,即: (c1 – b1)(c1 – a1) –(b1 – a1)(b1 + a1 – c1) = (b1 + a1 – c1)2 –(b1 – a1)(b1 + a1 – c1) =(b1 + a1 – c1)[(b1 + a1 – c1) –(b1 – a1)] =(b1 + a1 – c1)[2c1b1 + 2c1a1 – 2c12 – a1b1 + a12] =(b1 + a1 – c1)[2c1b1 + 2c1a1 – c12 – a12 –b12– a1b1 + a12] =(b1 + a1 – c1)[2c1b1 + 2c1a1 – c12 – b12 – b1a1]。 比較答案兩式,可知太虛弦加入和 = 兩個虛弦內少個平差。 內減和即平差也。 “和”即弦﹝在勾股形山川東 15﹞上勾股和=b15 + a15 。 b15 + a15 = (c1 – b1)(a1 – c1 + b1) +(c1 – b1)(a1 – c1 + b1) = (c1 – b1)(a1 – c1 + b1)(+) = (c1 – b1)(a1 – c1 + b1)(b1 + a1) 。 太虛弦內減和即: (c1 – b1)(c1 – a1) –(c1 – b1)(a1 – c1 + b1)(b1 + a1) = (a1 – c1 + b1)2 –(c1 – b1)(a1 – c1 + b1)(b1 + a1) =(a1 – c1 + b1)[c1(a1 – c1 + b1) – (c1 – b1)(b1 + a1)] =(a1 – c1 + b1)[(c1b1 + c1a1 – c12) – (c1b1 + c1a1 – b12 – a1b1)] =(a1 – c1 + b1)(c1b1 + c1a1 – c12 – c1b1 – c1a1 + b12 + a1b1) =(a1 – c1 + b1)(– c12 + b12 + a1b1) =(a1 – c1 + b1)( – a12 –b12 + b12 + a1b1) =(a1 – c1 + b1)( – a12 +a1b1) =(a1 + b1 – c1)(b1 – a1)。 “平差”指平弦﹝在勾股形月川青 8 或川地夕 9﹞上勾股較。 平弦上勾股較 = b8– a8 = (a1 + b1 – c1) –(a1 + b1 – c1) = (a1 + b1 – c1)(1 –) =(a1 + b1 – c1)(b1 – a1) =(b12 –a12 – c1b1 + c1a1) 。 比較兩式,可知太虛弦內減和 = 平差。 加入明二和共即極和內少個虛黃也。 已知“和”即弦上勾股和 = b15 +a15 = (c1 – b1)(a1 – c1 + b1)(b1 + a1) 。 “明和”即明弦勾股和 = b14 +a14 = (c1 – a1)(b1 – c1 + a1)(a1 + b1) 。 “二和”即“和”加“明和”。 太虛弦+ 二和,即: = (c1 – b1)(c1 – a1) + (c1 – b1)(a1 – c1 + b1)(b1 + a1) + (c1 – a1)(b1 – c1 + a1)(a1 + b1) = (a1 – c1 + b1)2 + (c1 – b1)(a1 – c1 + b1)(b1 + a1) + (c1 – a1)(b1 – c1 + a1)(a1 + b1) =(a1 – c1 + b1)[c1(a1 – c1 + b1) + (c1 – b1)(b1 + a1) + (c1 – a1)(a1 + b1)] =(a1 – c1 + b1)(c1b1 + c1a1 – c12 – 2a1b1 + 2c1b1 + 2c1a1 – a12 – b12) =(a1 – c1 + b1)(3c1b1 + 3c1a1 – 2c12 – 2a1b1)。 皇極在勾股形日川心 12。“極和”即皇極勾股和 = b12 +a12 “虛黃”即虛弦三事較 = 弦和較 = b13 + a13 – c13。 b13 + a13 – c13 =(c1 – a1)(c1 – b1)(a1 + b1 – c1) 。 極和內– 虛黃,即: (a1 + b1 – c1)(b1 + a1) –(c1 – a1)(c1 – b1)(a1 + b1 – c1) = (a1 + b1 – c1)[(b1 + a1) – (c1 – a1)(c1 – b1)] =(a1 – c1 + b1)(c1b1 + c1a1 + 2c1b1 + 2c1a1 – 2c12 – 2a1b1) =(a1 – c1 + b1)(3c1b1 + 3c1a1 – 2c12 – 2a1b1)。 比較答案兩式,可知相等,所以太虛弦 + 明二和共 = 極和內少個虛黃。 若減於明二和共即明股勾共也。 已知明二和共 = (c1 – b1)(a1 – c1 + b1)(b1 + a1) + 太虛弦若減於明二和共 = (c1 – b1)(a1 – c1 + b1)(b1 + a1) + =(a1 – c1 + b1)[(c1 – b1)(b1 + a1) + (c1 – a1)(a1 + b1) – c1(b1 + a1 – c1)] =(a1 – c1 + b1)[c1b1 + c1a1 – b12 – a1b1 + c1a1 + c1b1 – a12 – a1b1 – c1b1 – c1a1 + c12] =(a1 – c1 + b1)[c1b1 + c1a1 – 2a1b1] 。 已知東川勾﹝又稱勾﹞:a15 = (c1 – b1)(a1 – c1 + b1)。 日南股﹝又稱明股﹞:b14 = (c1 – a1)(b1 – c1 + a1)。 明股勾共 = (c1 – b1)(a1 – c1 + b1) + (c1 – a1)(b1 – c1 + a1) = (a1 + b1 – c1)[(c1 – b1) + (c1 – a1)] =(a1 – c1 + b1)(c1a1 – a1b1 + c1b1 – a1b1) =(a1 – c1 + b1)(c1a1 – 2a1b1 + c1b1) 。 比較兩式,可知太虛弦若減於明= 明股勾共。 減於髙弦即明弦。 已知髙弦﹝即天日或日山﹞:c6 = ( a1 +b1 – c1) 。 太虛弦若減於髙弦即 ( a1 +b1 – c1) – (a1 – c1 + b1)2 = ( a1 +b1 – c1)[1 –(a1 – c1 + b1)] = ( a1 +b1 – c1)(b1 – a1 – b1 + c1) = (c1 – a1)(b1 – c1 + a1)。 日月為明弦﹝簡稱明弦﹞:c14 = (c1 – a1)(b1 – c1 + a1)。 比較兩式,可知太虛弦減於髙弦 = 明弦。 減于平弦即弦。 已知平弦:c8 = (a1 + b1 – c1) 。 太虛弦減于平弦 = (a1 + b1 – c1) –(a1 – c1 + b1)2 = (a1 + b1 – c1)[1 –(a1 – c1 + b1)] = (a1 + b1 – c1)(a1 – a1 – b1 + c1) = (a1 + b1 – c1)(c1 – b1)。 已知山川弦﹝簡稱弦﹞:c15 =(c1 – b1)(a1 – c1 + b1)。 比較兩式,可知太虛弦減于平弦 = 弦。 加於角差即二明勾一極差也。 先說明何謂“角差”,據《測圓海鏡》所云“角差”乃髙股平勾差。 “髙股”在“天日旦”或“日山朱”之句股形;“平勾”在“月川青”或“川地夕”之句股形。宜先注意: 髙股:b6 = = (a1 + b1 – c1),髙勾:a6 = (a1 + b1 – c1)。 平勾:a8 = = (a1 + b1 – c1),平股:b8 = (a1 + b1 – c1)。 髙股平勾差 = b6 – a8 = (a1 + b1 – c1) –(a1 + b1 – c1) = (a1 + b1 – c1)[–] = (a1 + b1 – c1)。 以上之式是為“角差”。 太虛弦加於角差 = (a1 – c1 + b1)2 + (a1 + b1 – c1) = (a1 + b1 – c1)[c1(a1 – c1 + b1) + b12 – a12] = (a1 + b1 – c1)[c1b1 + c1a1 – c12 + b12 – a12] = (a1 + b1 – c1)[c1b1 + c1a1 – a12 – b12+ b12 – a12] = (a1 + b1 – c1)[c1b1 + c1a1 – 2a12]。 已知南月勾﹝又稱明勾,在勾股形日月南 14﹞: “極差”指皇極勾股較。 皇極勾股較= b12 – a12 = (a1 + b1 – c1) – (a1 + b1 – c1) = (a1 + b1 – c1)[–] = (a1 + b1 – c1)(b1 – a1)。 二明勾加一極差,即: = 2 × (c1 – a1)(b1 – c1 + a1) + (a1 + b1 – c1)(b1 – a1) = (a1 + b1 – c1)[2(c1 – a1) + (b1 – a1)] =(a1 – c1 + b1)(2a1c1 – 2a12+ c1b1 – c1a1) =(a1 – c1 + b1)(a1c1 – 2a12+ c1b1) 。 比較答案兩式,可知相等,所以太虛弦 + 角差 = 二明勾 +極差。 減於角差即一極差二股較也。 太虛弦減於角差 = (a1 – c1 + b1)2 – (a1 + b1 – c1) = (a1 + b1 – c1)[c1(a1 – c1 + b1) – b12 + a12] = (a1 + b1 – c1)[c1b1 + c1a1 – c12 – b12+ a12] = (a1 + b1 – c1)[c1b1 + c1a1 – a12 – b12 –b12 + a12] = (a1 + b1 – c1)[c1b1 + c1a1 – 2b12]。 已知山東股﹝又稱股﹞:b15 =(c1 – b1)(a1 – c1 + b1)。 一極差加二股較,即: 2 ×(c1 – b1)(a1 – c1 + b1) –(a1 + b1 – c1)(b1 – a1) = (a1 – c1 + b1)[2(c1 – b1) –(b1 – a1)] = (a1 – c1 + b1)(2b1c1 – 2b12 – c1b1 + c1a1) = (a1 – c1 + b1)(b1c1 – 2b12 + c1a1)。 比較兩式,可知太虛弦減於角差 = 一極差二股較。 得傍差即明股勾共。 依《測圓海鏡》所云,明二差較是為“傍差”﹝或作“旁差”﹞。 已知明差 = b14 – a14 = (c1 – a1)(b1 – c1 + a1) – (c1 – a1)(b1 – c1 + a1) = (c1 – a1)( a1 + b1 – c1)[–]。 差 = b15 – a15 = (c1 – b1)(a1 – c1 + b1) – (c1 – b1)(a1 – c1 + b1) =(c1 – b1)( a1 + b1 – c1) [–]。 二差較 = 明差 – 差 ,即: (c1 – a1)( a1 + b1 – c1)[–] – (c1 – b1)( a1 + b1 – c1) [–] = ( a1 +b1 – c1)[–][(c1 – a1) – (c1 – b1)] = (a1 + b1 – c1)(b1 – a1) =(a1 + b1 – c1) = (a1 + b1 – c1) = (a1 + b1 – c1) = (a1 + b1 – c1)。 以上之值即為“傍差”。 太虛弦得傍差即太虛弦加傍差: (a1 – c1 + b1)2 + (a1 + b1 – c1) =(a1 + b1 – c1)[c1(b1 + a1 – c1) + c12 – 2a1b1] =(a1 + b1 – c1)( c1b1 + c1a1 – c12+ c12 – 2a1b1) =(a1 + b1 – c1)(c1b1 + c1a1 – 2a1b1) 。 已知日南股﹝又稱明股﹞:b14 =(c1 – a1)(b1 – c1 + a1)。 東川勾﹝又稱勾﹞:a15 =(c1 – b1)(a1 – c1 + b1)。 明股勾共 = b14 + a15,即: (c1 – a1)(b1 – c1 + a1) + (c1 – b1)(a1 – c1 + b1) = (a1 + b1 – c1)[(c1 – a1) +(c1 – b1)] = (a1 + b1 – c1)(c1b1 – b1a1 + c1a1 – a1b1) =(a1 + b1 – c1)(c1b1 + c1a1 – 2a1b1) 。 比較答案兩式,可知太虛弦 + 傍差 = 明股 + 勾。 內減傍差即太虛三事和內去了極雙差也﹝按雙差係勾弦差股弦差﹞。 太虛弦內減傍差,即: (a1 – c1 + b1)2 –(a1 + b1 – c1) =(a1 + b1 – c1)[c1(b1 + a1 – c1) – c12 + 2a1b1] =(a1 + b1 – c1)( c1b1 + c1a1 – c12 – c12+ 2a1b1) =(a1 + b1 – c1)( c1b1 + c1a1 + 2a1b1 – 2c12) 。 “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 ﹝即圓直徑﹞。 皇極勾弦較即勾弦差,股弦較即股弦差。 皇極勾弦較 = c12 – a12 = (a1 + b1 – c1) –(a1 + b1 – c1) = (a1 + b1 – c1)[ – 1] = (a1 + b1 – c1)(c1 – a1) 。 皇極股弦較= c12 – b12 = (a1 + b1 – c1) – (a1 + b1 – c1) = (a1 + b1 – c1)[– 1] = (a1 + b1 – c1)(c1 – b1) 。 以上之和是為“極雙差”。三事和內去了極雙差,即: (a1 + b1 – c1) – (a1 + b1 – c1)(c1 – a1) –(a1 + b1 – c1)(c1 – b1) = (a1 + b1 – c1)[1 –(c1 – a1) –(c1 – b1)] =(a1 + b1 – c1)[2a1b1 + c1a1 – c12 + c1b1 – c12] =(a1 + b1 – c1)[2a1b1 + c1a1 – 2c12 + c1b1] 。 比較兩式,可知太虛弦內減傍差 = 太虛三事和內去極雙差。 內加虛差即二明勾。 太虛弦內加虛差 = c13 + (b13 – a13) = c13 + b13 – a13。 即太虛弦較和 = 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) 。 已知南月勾﹝又稱明勾﹞:a14 = (c1 – a1)(b1 – c1 + a1)。 二明勾 = 2 × (c1 – a1)(b1 – c1 + a1) = (c1 – a1)(b1 + a1 – c1) 。 比較兩式,可知太虛弦內加虛差 = 二明勾。 內減虛差即二股。 太虛弦內減虛差 = c13 – (b13 – a13)即弦較較。 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)。 已知山東股﹝又稱股,在勾股形山川東 15﹞:b15 二股 = 2 ×(c1 – b1)(a1 – c1 + b1) = (a1 + b1 – c1)(c1 – b1)。 比較兩式,可知太虛弦內減虛差 = 二股。 內加虛黃方即虛和。 從前圖可知,太虛弦與虛黃方同在一勾股形,可作如下之簡証: 已知虛黃方 =b13 + a13 – c13。 太虛弦內加虛黃方 = c13 + (b13 + a13 – c13) = c13 + b13 + a13 – c13 = b13 + a13 是為虛和。“虛和”指太虛勾股和 = b13 + a13。,即: b13 + a13 = (c1 – b1)(c1 – a1) +(c1 – b1)(c1 – a1) = (c1 – b1)(c1 – a1)[+] =(c1 – b1)(c1 – a1)(b1 + a1)。 內減虛黃方即太虛大小差併也。 太虛弦內減虛黃方 = c13 – (b13 + a13– c13) = c13 – b13 – a13 + c13= 2c13 – b13– a13。 已知太虛大差 = c13 – a13。 太虛大差即太虛勾弦較 = c13 – a13 = (c1 – b1)(c1 – a1)2。 已知太虛小差 = c13 – b13。 太虛小差即太虛股弦較 = c13 – b13 = (c1 – a1)(c1 – b1)2。 太虛大小差併 = c13 – a13 +c13 – b13 = 2c13 – b13 – a13。 所以太虛弦內減虛黃方 = 太虛大小差併 = 2c13– b13 – a13。 2c13 – b13 – a13 = (c1 – b1)(c1 – a1)2 + (c1 – a1)(c1 – b1)2 = (c1 – b1)(c1 – a1)[(c1 – a1) + (c1 – b1)] = (c1 – b1)(c1 – a1)(2c1 – a1 – b1)。 以下為《測圓海鏡細草》原文: |
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