《測圓海鏡》之極差等式﹝諸差3﹞ 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。本文著重皇極勾股較﹝即“極差”﹞之相關等式。 關鍵詞:極差、旁差、角差、蓌和、蓌差 《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 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”。 注意圓徑為 a1 + b1 – c1,見上圖之東南西北圓。 本文主要談及十五勾股形有關三邊相差之等式,其中部分等式曾在“五和五較”等式中出現,可參閱筆者相關之文章。 注意等式 (c1 – b1)(c1 – a1) = (a1 + b1 – c1)2。 本文取自《測圓海鏡‧卷一‧諸差》。筆者有以下文涉及〈諸差〉: 《測圓海鏡》之大差差、小差差等式﹝諸差1﹞ 《測圓海鏡》之髙差、旁差、極雙差等式﹝諸差2﹞ 本文乃以上二文之延續。 以下為有關“極差”及相關之等式: 極差內加旁差為大差差。內減旁差為小差差也。內加虛差即角差。內減虛差即次差也。倍極差為大差差小差差共,則倍旁差為之較。倍極弦為大差弦小差弦共,倍極差為之較。以極差為明差平差共,則以蓌差為之較。以極差為髙差差共,則以蓌和為之較。副置蓌和上加蓌差而半之即旁差也。減蓌差而半之則虛差也。極差內減二之平差得蓌差。 以下為各條目之証明: 極差內加旁差為大差差。 “極差”指皇極勾股較﹝在勾股形日川心 12,可參閱上兩圖﹞。 已知皇極勾股較 = b12 – a12= (a1 + b1 – c1) – (a1 + b1 – c1) = (a1 + b1 – c1)[–] = (a1 + b1 – c1)(b1 – a1) 。 “旁差”又名“傍差”,據《測圓海鏡》所云,“明二差較”是為傍差,此亦為“旁差”定義。 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) [–]。 旁差 = 二差較 = 明差 – 差 = (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) 。 以上之式是為“傍差” 。 極差內加旁差,即: (a1 + b1 – c1)(b1 – a1) + (a1 + b1 – c1) = (a1 + b1 – c1)(b1 – a1)[c1 + (b1 – a1)] = (a1 + b1 – c1)(b1 – a1)[b1 – a1+ c1] = (b1 – a1)[b1 – (c1 – a1)][b1 + (c1– a1)] = (b1 – a1)[b12 – (c1 – a1)2] = (b1 – a1)[b12 – c12 – a12 + 2c1a1] = (b1 – a1)[b12 – a12 –b12 – a12 + 2c1a1] = (b1 – a1)(2c1a1 – 2a12) = (b1 – a1)(c1 – a1) #。 “大差差”指大差上勾股較,勾股較即勾股差﹝在勾股形天月坤 10﹞。 大差上勾股差 = b10 – a10 = (c1 – a1) –(c1 – a1) = (c1 – a1)(1 – ) = (c1 – a1)(b1 – a1) #。 比較兩式可知相同,所以極差內加旁差 = 大差差。 內減旁差為小差差也。 指極差內減旁差,即極差 –旁差,即: (a1 + b1 – c1)(b1 – a1) – (a1 + b1 – c1) = (a1 + b1 – c1)(b1 – a1)[c1 – (b1 – a1)] = (a1 + b1 – c1)(b1 – a1)[– b1 + a1 + c1] = (b1 – a1)[a1 – (c1 – b1)][a1 + (c1– b1)] = (b1 – a1)[a1 – (c1 – b1)][a1 + (c1– b1)] = (b1 – a1)[a12 – (c1 – b1)2] = (b1 – a1)[a12 – c12 – b12 + 2c1b1] = (b1 – a1)[a12 – a12 –b12 – b12 + 2c1b1] = (b1 – a1)[ – 2b12 + 2c1b1] = (b1 – a1)(c1 – b1) #。 “小差差”指小差﹝在勾股形山地艮 11﹞上勾股較。 小差上勾股較 = – (c1 – b1) + (c1 – b1) = (c1 – b1)(– 1) = (c1 – b1)(b1 – a1) #。 比較答案兩式可知相等,所以極差內減旁差 = 小差差。 內加虛差即角差。 “虛差”指太虛勾股較﹝在勾股形月山泛 13﹞。 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)。 極差內加虛差,即: (a1 + b1 – c1)(b1 – a1) +(c1 – b1)(c1 – a1)(b1 – a1) = (a1 + b1 – c1)(b1 – a1) + (a1 + b1 – c1)2(b1 – a1) =(a1 + b1 – c1)(b1 – a1)[c1 + a1 + b1 – c1] =(a1 + b1 – c1)(b1 – a1)[a1 + b1] = (a1 + b1 – c1) #。 據《測圓海鏡》所云,髙股平勾差名為“角差”。 髙股:b6 = = (a1 + b1 – c1), 平勾:a8 = = (a1 + b1 – c1)。 髙股平勾差 = b6 – a8 = (a1 + b1 – c1) –(a1 + b1 – c1) = (a1 + b1 – c1)[–] = (a1 + b1 – c1) #。 以上是為“角差”或稱為“逺差”。 比較答案兩式,可知極差內加虛差 = 角差。 內減虛差即次差也。 本條指極差內減虛差﹝極差與虛差見前條﹞,即: (a1 + b1 – c1)(b1 – a1) – (c1 – b1)(c1 – a1)(b1 – a1) = (a1 + b1 – c1)(b1 – a1) – (a1 + b1 – c1)2(b1 – a1) =(a1 + b1 – c1)(b1 – a1)[c1 – a1 – b1 + c1] =(a1 + b1 – c1)(b1 – a1)[2c1 – a1 – b1] #。 據《測圓海鏡》所云,明二差共名次差,又名近差,又名戾﹝音列﹞和。 明差指明勾與明股之差。 明差 = b14 – a14 = (c1 – a1)(b1 – c1 + a1) – (c1 – a1)(b1 – c1 + a1) = (c1 – a1)( a1 + b1 – c1)[–]。 差指勾與股之差。 差 = b15 – a15 = (c1 – b1)(a1 – c1 + b1) – (c1 – b1)(a1 – c1 + b1) =(c1 – b1)( a1 + b1 – c1)[–]。 二差共 = 明差 + 差 = (c1 – a1)( a1 + b1 – c1)[–] + (c1 – b1)( a1 + b1 – c1) [–] = ( a1 +b1 – c1)[–](c1 – a1 + c1 – b1) = (a1 + b1 – c1)(2c1 – a1 – b1) #。 上式是為次差,故明二差共得次差。 倍極差為大差差小差差共。 “極差”指皇極勾股較。 皇極勾股較= b12 – a12 = (a1 + b1 – c1) – (a1 + b1 – c1) = (a1 + b1 – c1)(b1 – a1)。 倍極差= 2 ×(a1 + b1 – c1)(b1 – a1) =(a1 + b1 – c1)(b1 – a1) #。 “大差差”指大差上勾股較 = (c1 – a1)(b1 – a1) 。 “小差差”指小差上勾股較 = (c1 – b1)(b1 – a1)。以上兩式見前條。 大差差小差差共,即: (c1 – a1)(b1 – a1) + (c1 – b1)(b1 – a1) = (b1 – a1)[(c1 – b1) +(c1 – a1)] = (b1 – a1)[b1(c1 – b1) + a1(c1 – a1)] = (b1 – a1)[b1c1 – b12+ a1c1 – a12] = (b1 – a1)[b1c1 + a1c1 – c12] = (b1 – a1)c1(a1 + b1 – c1) = (a1 + b1 – c1)(b1 – a1) #。 比較答案兩式可知相等,所以倍極差 =大差差 + 小差差。 則倍旁差為之較。 “傍差”= (a1 + b1 – c1) ﹝見前﹞。 倍旁差 = 2 × (a1 + b1 – c1) = (a1 + b1 – c1) #。 大差差小差差較,即: (c1 – a1)(b1 – a1) – (c1 – b1)(b1 – a1) = (b1 – a1)[ –(c1 – b1) +(c1 – a1)] = (b1 – a1)[– b1(c1 – b1) + a1(c1 – a1)] = (b1 – a1)[– b1c1 + b12+ a1c1 – a12] = (b1 – a1)[(b1 – a1)(b1 + a1) – c1(b1 – a1)] = (b1 – a1)2[b1 + a1 – c1] = (a1 + b1 – c1) #。 比較答案兩式可知相等,所以倍旁差 =大差差小差差較。 倍極弦為大差弦小差弦共。 已知皇極弦﹝日川﹞:c12 = (a1 + b1 – c1) 。 倍極弦= 2 × (a1 + b1 – c1) = (a1 + b1 – c1) #。 已知大差弦﹝在勾股形天月坤 10﹞=c10 = (c1 – a1) 。 小差弦﹝在勾股形山地艮 11﹞= c11 = (c1 – b1) 。 大差弦小差弦共= c10 + c11 = (c1 – a1) + (c1 – b1) = c1[(c1 – a1) +(c1 – b1)] = [a1(c1 – a1) + b1(c1 – b1)] = [a1c1 – a12+ b1c1 – b12] = [a1c1 + b1c1 – c12] = (a1 + b1 – c1) #。 比較答案兩式可知相等,所以倍極弦 = 大差弦 + 小差弦。 倍極差為之較。 “極差”指皇極勾股較。 皇極勾股較= b12 – a12 = (a1 + b1 – c1)(b1 – a1)。 倍“極差”= 2 ×(a1 + b1 – c1)(b1 – a1) = (a1 + b1 – c1)(b1 – a1) #。 大差弦小差弦較 = c10– c11 = (c1 – a1) – (c1 – b1) = c1[(c1 – a1) –(c1 – b1)] = [a1(c1 – a1) – b1(c1 – b1)] = [a1c1 – a12 – b1c1 + b12] = [(b1 + a1)(b1 – a1) – c1(b1 – a1)] = (b1 – a1)(a1 + b1 – c1) #。 比較兩式可知相同,所以倍“極差”= 大差弦小差弦較。 以極差為明差平差共。 極差即皇極勾股較 = (a1 + b1 – c1)(b1 – a1) #。 “明差”指明弦勾股較﹝在勾股形日月南 14﹞。 明弦勾股較=b14 – a14= (c1 – a1)(b1 – c1 + a1) –(c1 – a1)(b1 – c1 + a1) = (c1 – a1)(b1 – c1 + a1)[–] =(c1 – a1)(b1 – c1 + a1)(b1 – a1) 。 “平差”指平弦上勾股較﹝在勾股形月川青 8 或川地夕 9﹞。 平弦上勾股較 = b8– a8 = (a1 + b1 – c1) –(a1 + b1 – c1) = (a1 + b1 – c1)(1 –) =(a1 + b1 – c1)(b1 – a1) 。 明差平差共,即: (c1 – a1)(b1 – c1 + a1)(b1 – a1) + (a1 + b1 – c1)(b1 – a1) = (a1 + b1 – c1)(b1 – a1)[(c1 – a1) + 1] = (a1 + b1 – c1)(b1 – a1)[(c1 – a1) +a1] = (a1 + b1 – c1)(b1 – a1) #。 比較答案兩式可知相等,所以極差 = 明差 + 平差。 則以蓌差為之較。 明差平差較,即: (c1 – a1)(b1 – c1 + a1)(b1 – a1) – (a1 + b1 – c1)(b1 – a1) = (a1 + b1 – c1)(b1 – a1)[(c1 – a1) – 1] = (a1 + b1 – c1)(b1 – a1)[(c1 – a1) – a1] = (a1 + b1 – c1)(b1 – a1)(c1 – 2a1) #。 據《測圓海鏡》所云,虛差不及傍差名“蓌差”,此即“蓌差”之定義。 已知虛勾 = a13 =(c1 – a1)(c1 – b1),虛股 = b13 =(c1 – a1)(c1 – b1)。 虛差 = b13 – a13 = (c1 – a1)(c1 – b1) – (c1 – a1)(c1– b1) = (c1 – a1)(c1– b1)[–]。 虛差不及傍差,即傍差 – 虛差,即: (a1 + b1 – c1) – (c1 – a1)(c1 – b1)[–] = (a1 + b1 – c1) –( a1 + b1 – c1)2 = (a1 + b1 – c1)[c12 – 2a1b1 – (b1 – a1)[(b1 + a1) – c1]] = (a1 + b1 – c1){(b1 – a1)2 –(b1 – a1)[(b1 + a1) – c1]} = (a1 + b1 – c1)(b1 – a1){b1 – a1– [(b1 + a1) – c1]} = (a1 + b1 – c1)(b1 – a1){b1 – a1– b1 – a1 + c1} = (a1 + b1 – c1)(b1 – a1)(c1 – 2a1) #。 以上之式是為“蓌差”。 比較兩式可知相同,所以明差平差較 = 蓌差。 以極差為髙差、差共。 極差即皇極勾股較 = (a1 + b1 – c1)(b1 – a1) #﹝見前條﹞。 “髙差”指髙弦上勾股較﹝在勾股形天日旦 6 或日山朱7﹞。 髙弦上勾股較= b6 – a6 = (a1 + b1 – c1) – (a1 + b1 – c1) = (a1 + b1 – c1)(– 1) = (a1 + b1 – c1)(b1 – a1) 。 “差”指弦上勾股較﹝在勾股形山川東 15﹞。 弦上勾股較 = b15 – a15 = (c1 – b1)(a1 – c1 + b1) – (c1 – b1)(a1 – c1 + b1) = (c1 – b1)(a1 – c1 + b1)(–) = (c1 – b1)(a1 – c1 + b1)(b1 – a1) 。 髙差差共,即: = (a1 + b1 – c1)(b1 – a1) + (c1 – b1)(a1 – c1 + b1)(b1 – a1) = (a1 + b1 – c1)(b1 – a1)[1 + (c1 – b1)] = (a1 + b1 – c1)(b1 – a1)[b1 + (c1 – b1)] = (a1 + b1 – c1)(b1 – a1) #。 比較兩式可知相同,所以極差 =髙差 + 差。 則以蓌和為之較。 本條之“較”指髙差差較,即: = (a1 + b1 – c1)(b1 – a1) – (c1 – b1)(a1 – c1 + b1)(b1 – a1) = (a1 + b1 – c1)(b1 – a1)[1 – (c1 – b1)] = (a1 + b1 – c1)(b1 – a1)[b1 – (c1 – b1)] = (a1 + b1 – c1)(b1 – a1)(b1 – c1 + b1) = (a1 + b1 – c1)(b1 – a1)(2b1 – c1) #。 據《測圓海鏡》,蓌和即傍差 + 虛差,即: (a1 + b1 – c1) + (c1 – a1)(c1 – b1)[–] = (a1 + b1 – c1) +( a1 +b1 – c1)2 = (a1 + b1 – c1)[c12 – 2a1b1 + (b1 – a1)[(b1 + a1) – c1]] = (a1 + b1 – c1){(b1 – a1)2 + (b1 – a1)[(b1 + a1) – c1]} = (a1 + b1 – c1)(b1 – a1){b1 – a1 + [(b1 + a1) – c1]} = (a1 + b1 – c1)(b1 – a1){b1 – a1 + b1 + a1 – c1} = (a1 + b1 – c1)(b1 – a1)(2b1 – c1) #。 以上是為“蓌和”之式。 所以髙差差較 = “蓌和”。 副置蓌和上加蓌差而半之即旁差也。 此處之“副置”疑指另外其他之算法。 蓌和上加蓌差,即: (a1 + b1 – c1)(b1 – a1)(2b1 – c1) + (a1 + b1 – c1)(b1 – a1)(c1 – 2a1) = (a1 + b1 – c1)(b1 – a1)[(2b1 – c1) + (c1 – 2a1)] = (a1 + b1 – c1)(b1 – a1)(2b1 – 2a1) = (a1 + b1 – c1)(b1 – a1)2。 “半之”即除以 2 = (a1 + b1 – c1)(b1 – a1)2 # 。 又已知“傍差”= (a1 + b1 – c1) #﹝見前條﹞。 比較兩式可知相同,所以蓌和上加蓌差而半之即旁差。 減蓌差而半之則虛差也。 蓌和上減蓌差,即: (a1 + b1 – c1)(b1 – a1)(2b1 – c1) – (a1 + b1 – c1)(b1 – a1)(c1 – 2a1) = (a1 + b1 – c1)(b1 – a1)[(2b1 – c1) – (c1 – 2a1)] = (a1 + b1 – c1)(b1 – a1)(2b1 + 2a1 – 2c1) = (a1 + b1 – c1)(b1 – a1)(a1 + b1 – c1) = (a1 + b1 – c1)2(b1 – a1)。 “半之”即除以 2 = (a1 + b1 – c1)2(b1 – a1), 但 (a1 + b1 – c1)2(b1 – a1) =(c1 – b1)(c1 – a1)(b1 – a1) #。 注意等式 (c1 – b1)(c1 – a1) = (a1 + b1 – c1)2。 “虛差”指太虛勾股較﹝在勾股形月山泛 13﹞。 太虛勾股較 = b13 – a13 = (c1 – b1)(c1 – a1)(b1 – a1) #。 比較兩式可知相同,蓌和上減蓌差而半之 = 虛差。 極差內減二之平差得蓌差。 極差﹝在勾股形日川心 12﹞即皇極勾股較 = (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)(b1 – a1) 。 二之平差即2 × (a1 + b1 – c1)(b1 – a1) = (a1 + b1 – c1)(b1 – a1) 。 極差內減二之平差得: (a1 + b1 – c1)(b1 – a1) –(a1 + b1 – c1)(b1 – a1) =(a1 + b1 – c1)(b1 – a1)[– 1] =(a1 + b1 – c1)(b1 – a1)(c1 – 2a1) #。 從上題可知“蓌差”= (a1 + b1 – c1)(b1 – a1)(c1 – 2a1) #。 比較兩式,可知極差內減二之平差得蓌差。 以下為《測圓海鏡細草》原文: |
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