《測圓海鏡》﹝諸差7﹞之明雙差、雙差等式說 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》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。本文著重明雙差、雙差之相關等式及其証明。 關鍵詞:明雙差、雙差、明差、虛黃、極雙差 《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。圓徑乃十五勾股形三邊重要因子之一,其他因子為最大勾股形之勾股較、勾弦較及股弘較。 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﹝雙差積圓徑平方半等式﹞。 本文取自《測圓海鏡‧卷一‧諸差》。筆者有以下之文涉及〈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﹞ 《測圓海鏡》之極差等式﹝諸差3﹞ 《測圓海鏡》之角差及虛差等式說﹝諸差 4﹞ 《測圓海鏡》﹝諸差5﹞之髙差、黃廣及黃長等式 《測圓海鏡》﹝諸差6﹞之邊弦、底弦等式說 本文乃以上六文之延續。 以下為有關“明雙差”及“雙差”相關之等式: 明雙差亦為明二大差。其較則明差也。雙差亦為明二小差。其較則差也。明雙差內減明差即虛黃。雙差上加差亦同上。以明雙差加明和即兩明弦。以雙差加和則兩弦也。以明雙差減明和而半之即明黃又為虛大差。以雙差減於和而半之即黃,又為虛小差也。以虛大差減明和即為明弦。以虛小差減和即弦也。明雙差雙差相較則次差也。明雙差雙差相併,加於明二和共則為兩個極雙差。若以減於明二和共則為兩個虛雙差也。明雙差上加虛雙差即明二股共。雙差上加虛雙即明二勾共也。 以下為各條目之証明: 明雙差亦為明二大差。 “明雙差”指明弦勾弦較與明弦股弦較之和。明弦在勾股形日月南 14。 明弦勾弦較 = c14 – a14 =(b1 – c1 + a1) (c1 – a1)2。 明弦股弦較 = c14 – b14 =(c1 – a1)(b1 – c1 + a1)(c1 – b1) 。 所以明雙差,即: (b1 – c1 + a1) (c1 – a1)2 + (c1 – a1)(b1 – c1 + a1)(c1 – b1) =(b1 – c1 + a1) (c1 – a1)[(c1 – a1) + (c1 – b1)] =(b1 – c1 + a1) (c1 – a1) (2c1 – a1 – b1) #。 明大差即明弦勾弦較 =(b1 – c1 + a1) (c1 – a1)2。 大差即弦上勾弦較 = c15 – a15。 c15 – a15 = (c1 – b1)(a1 – c1 + b1) – (c1 – b1)(a1 – c1 + b1) = (c1 – b1)(c1 – a1)(b1 + a1 – c1) = 明弦股弦較。 明大差+ 大差 = (b1 – c1 + a1) (c1 – a1)2 + (c1 – b1)(c1 – a1)(b1 + a1 – c1) = (b1 – c1 + a1)(c1 – a1)[(c1 – a1) + (c1 – b1)] =(b1 – c1 + a1) (c1 – a1) (2c1 – a1 – b1) #。 所以明弦勾弦較+ 明弦股弦較 = 明大差 + 大差。 其較則明差也。 “其較”指明弦勾弦較與明弦股弦較之差,即: (b1 – c1 + a1) (c1 – a1)2 – (c1 – a1)(b1 – c1 + a1)(c1 – b1) =(b1 – c1 + a1) (c1 – a1)[(c1 – a1) – (c1 – b1)] =(b1 – c1 + a1) (c1 – a1)(b1 – a1) #。 “明差”指明弦勾股較。 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) #。 比較兩式可知相同,所以明雙差與明二大差之差 = 明差。 雙差亦為明二小差。 “雙差”指弦上勾弦較加弦上股弦較。 弦上勾弦較 = c15 – a15 = (c1 – b1)(a1 – c1 + b1) – (c1 – b1)(a1 – c1 + b1) = (c1 – b1)(a1 – c1 + b1)[ – 1] = (c1 – b1)(b1 + a1 – c1)[c1 – a1] = (c1 – b1)(c1 – a1)(b1 + a1 – c1) 。 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) 。 弦上雙差 = (c1 – b1)(c1 – a1)(b1 + a1 – c1) + (c1 – b1)2(a1 – c1 + b1) = (c1 – b1)(b1 + a1 – c1)[(c1 – a1) + (c1 – b1)] =(c1 – b1)(b1 + a1 – c1)(2c1 – a1 – b1) #。 明小差即明弦股弦較 = c14 – b14 。 c14 – b14= (c1 – a1)(b1 – c1 + a1) – (c1 – a1)(b1 – c1 + a1) = (c1 – a1)(b1 – c1 + a1)[– 1] =(c1 – a1)(b1 – c1 + a1)(c1 – b1) 。 “小差”指弦上股弦較 = 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) 。 明小差 + 小差,即: (c1 – a1)(b1 – c1 + a1)(c1 – b1) + (c1 – b1)2(a1 – c1 + b1) = (b1 – c1 + a1)(c1 – b1)[(c1 – a1) + (c1 – b1)] = (b1 – c1 + a1)(c1 – b1)(2c1 – a1 – b1) #。 比較兩式可知相同,所以雙差 = 明二小差。 其較則差也。 “其較”指雙差之較 = 勾弦較 – 股弦較 = (c15 – a15) – (c15 – b15) = c15 – a15 – c15 + b15 = b15 – a15。 以上即差,即弦上勾股較。 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) #。 明雙差內減明差即虛黃。 明雙差= 明弦股弦較 + 明弦勾弦較 = (c14 – b14) + (c14 – a14); 明差= b14 – a14; 明雙差內減明差即(c14 – b14) + (c14 – a14)– (b14 – a14) = c14 – b14 + c14– a14 – b14 + a14 = 2(c14 – b14) = 2 ×(c1 – a1)(b1 – c1 + a1)(c1 – b1) ﹝見前條﹞ =(c1 – a1)(b1 – c1 + a1)(c1 – b1) 。 “虛黃”即太虛弦三事較 = 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) ﹝雙差積圓徑平方半等式﹞。 所以明雙差內減明差 = 虛黃。 雙差上加差亦同上。 雙差上加差 = 勾弦較 + 股弦較 + 勾股較 = (c15 – a15) + (c15 – b15) + (b15 – a15) = c15 – a15 +c15 – b15 + b15 – a15 = 2(c15 – a15) = 2 ×(c1 – b1)(c1 – a1)(b1 + a1 – c1)﹝見前條﹞ = (c1 – b1)(c1 – a1)(b1 + a1 – c1)。 此式同上。 以明雙差加明和即兩明弦。 已知明弦﹝在勾股形日月南 14﹞勾弦較=c14 – a14。 c14 – a14 = (c1 – a1)(b1 – c1 + a1) –(c1 – a1)(b1 – c1 + a1) =(c1 – a1)(b1 – c1 + a1)[– 1] =(b1 – c1 + a1)(c1 – a1)(c1 – a1) =(b1 – c1 + a1) (c1 – a1)2。 明弦股弦較 = c14 – b14 = (c1 – a1)(b1 – c1 + a1) – (c1 – a1)(b1 – c1 + a1) = (c1 – a1)(b1 – c1 + a1)[– 1] =(c1 – a1)(b1 – c1 + a1)(c1 – b1) 。 所以明雙差 =(b1 – c1 + a1) (c1 – a1)2 + (c1 – a1)(b1 – c1 + a1)(c1 – b1) =(b1 – c1 + a1) (c1 – a1)[(c1 – a1) + (c1 – b1)] =(b1 – c1 + a1) (c1 – a1)(2c1 – a1 – b1)。 已知“明和”即明弦勾股和。 明弦勾股和=b14 + a14 = (c1 – a1)(b1 – c1 + a1) +(c1 – a1)(b1 – c1 + a1) = (c1 – a1)(b1 – c1 + a1)[+] =(c1 – a1)(b1 – c1 + a1)(a1 + b1) 。 明雙差加明和 =(c1 – a1)(b1 – c1 + a1)(a1 + b1) + (b1 – c1 + a1)(c1 – a1)(2c1 – a1 – b1) = (b1 – c1 + a1)(c1 – a1)[(a1 + b1) + (2c1 – a1 – b1)] = (b1 – c1 + a1)(c1 – a1) × 2c1 =(c1 – a1)(b1 – c1 + a1)。 已知明弦 = c14 =(c1 – a1)(b1 – c1 + a1)。 兩明弦 = (c1 – a1)(b1 – c1 + a1)。 所以以明雙差加明和 = 兩明弦。 另法: 明雙差+ 明和 = 明弦勾弦較 + 明弦股弦較 + 明弦勾股和 = (c14 – a14) + (c14 –b14) + (a14 + b14) = c14 – a14 +c14 – b14+ a14 + b14 = 2c14。 以上即為兩明弦。 以雙差加和則兩弦也。 雙差 + 和 = 弦勾弦較 +弦股弦較 + 弦勾股和 = (c15 – a15) + (c15 – b15) + (a15 + b15) = c15 – a15 +c15 – b15+ a15 + b15 = 2c15。 以上即為兩弦。其實以上之形式任何勾股形皆適用。 以明雙差減明和而半之即明黃,又為虛大差。 以明雙差減明和而半之 = (明和 – 明雙差) = [明和 – (明弦勾弦較+ 明弦股弦較)] = {(a14+ b14)– [(c14 – a14) + (c14 – b14)]} = [a14 + b14 – (c14 – a14) – (c14 – b14)] = (a14 + b14 – c14 + a14– c14 + b14) = a14 + b14– c14。 以上即為明黃,或稱之為明黃方,又名明弦三事較,又名明弦弦和較。 明弦三事較 = 弦和較 =b14 + a14 – c14。 b14 + a14 – c14 = –(c1 – a1)(b1 – c1 + a1) + (c1 – a1)(b1 – c1 + a1) + =(c1 – a1)(b1 – c1 + a1)[ –++] =(c1 – a1)(b1 – c1 + a1)( – c1 + b1 + a1) =(c1 – a1)(b1 – c1 + a1)2 = (c1 – a1)(c1 – b1)(c1 – a1) = (c1 – b1)(c1 – a1)2 #。 “虛大差”即太虛勾弦較。 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 #。 所以以明雙差減明和而半之 = 明黃 = 虛大差。 以雙差減於和而半之即黃,又為虛小差也。 以雙差減和而半之,即: (和 – 雙差) = [和 – (弦勾弦較 +弦股弦較)] = {(a15+ b15)– [(c15 – a15) + (c15 – b15)]} = [a15 + b15 – (c15 – a15) – (c15 – b15)] = (a15 + b15 – c15 + a15– c15 + b15) = a15 + b15– c15。 以上即為黃。黃即弦三事較 弦上三事較即弦上弦和較 = (b15 + a15) – c15 = b15 + a15 – c15。 b15 + a15 – c15 = –(c1 – b1)(a1 – c1 + b1) +(c1 – b1)(a1 – c1 + b1) = (c1 – b1)(a1 – c1 + b1)[ –+ +] =(c1 – b1)(a1 – c1 + b1)( – c1 + b1 + a1) =(c1 – b1)(a1 – c1 + b1)2 = (c1 – a1)(c1 – b1)(c1 – b1) ﹝雙差積圓徑平方半等式﹞ =(c1 – b1)2(c1 – a1) #。 虛小差即太虛股弦較 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 #。 比較兩式,可知弦上三事較 = 太虛上股弦較。 以虛大差減明和即為明弦。 已知虛大差即太虛勾弦較,明和即明弦勾股和。 明弦勾股和=b14 + a14 = (c1 – a1)(b1 – c1 + a1) +(c1 – a1)(b1 – c1 + a1) = (c1 – a1)(b1 – c1 + a1)[+] =(c1 – a1)(b1 – c1 + a1)(a1 + b1) 。 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) =(b1 – c1 + a1)2(c1 – a1)。 虛大差減明和,即: (c1 – a1)(b1 – c1 + a1)(a1 + b1) –(b1 – c1 + a1)2(c1 – a1) = (c1 – a1)(b1 – c1 + a1)[(a1 + b1) – (b1 – c1 + a1)] = (c1 – a1)(b1 – c1 + a1)(a1 + b1 – b1 + c1 – a1) = (c1 – a1)(b1 – c1 + a1) × c1 = (c1 – a1)(b1 – c1 + a1) #。 日月為明弦﹝簡稱明弦﹞:c14 = (c1 – a1)(b1 – c1 + a1) #。 所以以虛大差減明和 = 明弦。 以虛小差減和即弦也。 “虛小差”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 = (a1 – c1 + b1)2(c1 – b1) ﹝雙差積圓徑平方半等式﹞。 “和”指弦上勾股和 = 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) 。 以虛小差減和,即: (c1 – b1)(a1 – c1 + b1)(b1 + a1) –(a1 – c1 + b1)2(c1 – b1) =(c1 – b1)(a1 – c1 + b1)[(b1 + a1) – (a1 – c1 + b1)] =(c1 – b1)(a1 – c1 + b1)(b1 + a1 – a1 + c1 – b1) =(c1 – b1)(a1 – c1 + b1) × c1 = (c1 – b1)(a1 – c1 + b1) #。 已知山川弦﹝簡稱弦﹞:c15 =(c1 – b1)(a1 – c1 + b1) #。 所以以虛小差減和 = 弦也。 明雙差雙差相較則次差也。 已知明雙差 = (b1 – c1 + a1)(c1 – a1)(2c1 – a1 – b1)。 弦上勾弦較 = c15 – a15 = (c1 – b1)(a1 – c1 + b1) – (c1 – b1)(a1 – c1 + b1) = (c1 – b1)(a1 – c1 + b1)[ – 1] = (c1 – b1)(b1 + a1 – c1)[c1 – a1] = (c1 – b1)(c1 – a1)(b1 + a1 – c1) 。 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) 。 弦上雙差,即: (c1 – b1)(c1 – a1)(b1 + a1 – c1) + (c1 – b1)2(a1 – c1 + b1) = (c1 – b1)(b1 + a1 – c1)[(c1 – a1) + (c1 – b1)] =(c1 – b1)(b1 + a1 – c1)(2c1 – a1 – b1)。 明雙差雙差較,即: (b1 – c1 + a1) (c1 – a1) (2c1 – a1 – b1) – =(b1 + a1 – c1)(2c1 – a1 – b1)[(c1 – a1) – (c1 – b1)] =(a1 + b1 – c1)(2c1 – a1 – b1)[c1 – a1 – c1 + b1] =(a1 + b1 – c1)(2c1 – a1 – b1)(b1 – a1) #。 明二差共名次差。 明差指明勾與明股之差,明勾與明股之式見前。 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,勾與股之式亦見前。 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。故明雙差雙差相較= 次差。 明雙差雙差又相併,加於明二和共,則為兩個極雙差。 明雙差雙差共 =(b1 – c1 + a1) (c1 – a1) (2c1 – a1 – b1) + =(b1 + a1 – c1)(2c1 – a1 – b1)[(c1 – a1) + (c1 – b1)] =(a1 + b1 – c1)(2c1 – a1 – b1)[c1 – a1 – b1 + c1] =(a1 + b1 – c1)(2c1 – a1 – b1)2。 已知明弦勾股和是為明和 = b14 + a14 。 b14 + a14 = (c1 – a1)(b1 – c1 + a1) +(c1 – a1)(b1 – c1 + a1) = (c1 – a1)(b1 – c1 + a1)[+] =(c1 – a1)(b1 – c1 + a1)(a1 + b1) 。 弦上勾股和是為和 = 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) 。 明二和共,即: (c1 – a1)(b1 – c1 + a1)(a1 + b1) + (c1 – b1)(a1 – c1 + b1)(b1 + a1) = (b1 – c1 + a1)(a1 + b1)[ (c1 – a1) + (c1 – b1)] = (b1 – c1 + a1)(a1 + b1)(2c1 – a1 – b1) 。 明雙差雙差相並,加於明二和共,即: (a1 + b1 – c1)(2c1 – a1 – b1)2+ (b1 – c1 + a1)(a1 + b1)(2c1 – a1 – b1) = (a1 + b1 – c1)(2c1 – a1 – b1)(2c1 – a1 – b1 + a1 + b1) = (a1 + b1 – c1)(2c1 – a1 – b1) × 2c1 = (a1 + b1 – c1)(2c1 – a1 – b1) #。 以下為極勾弦差及股弦差: c12 – a12 =(a1 + b1 – c1) –(a1 + b1 – c1) = (a1 + b1 – c1)[– 1], c12 – b12 =(a1 + b1 – c1) –(a1 + b1 – c1) = (a1 + b1 – c1)[– 1], 極雙差 = 即以上兩式之和,即: (a1 + b1 – c1)[– 1] + (a1 + b1 – c1)[– 1] = (a1 + b1 – c1)(c1 – a1) + (a1 + b1 – c1)(c1 – b1) = (a1 + b1 – c1)(c1 – a1 + c1 – b1) = (a1 + b1 – c1)(2c1 – a1 – b1)。 兩個極雙差 ﹝上式乘以2﹞= (a1 + b1 – c1)(2c1 – a1 – b1) #。 所以明雙差 +雙差 + 明二和共 = 兩個極雙差。 若以減於明二和共則為兩個虛雙差也。 明雙差雙差相併,減於明二和共,即: (b1 – c1 + a1)(a1 + b1)(2c1 – a1 – b1) –(a1 + b1 – c1)(2c1 – a1 – b1)2 = (a1 + b1 – c1)(2c1 – a1 – b1)( a1 + b1 – 2c1 + a1 + b1) = (a1 + b1 – c1)2(2c1 – a1 – b1) × 2 = (a1 + b1 – c1)2(2c1 – a1 – b1) #。 “虛雙差”即太虛勾弦較與太虛股弦較之和﹝在勾股形月山泛 13﹞。 已知太虛勾弦較 = 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。 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。 所以虛雙差 = (c1 – b1)(c1 – a1)2 + (c1 – a1)(c1 – b1)2 = (c1 – a1)(c1 – b1)[(c1 – a1) + (c1 – b1)] = (c1 – a1)(c1 – b1)(2c1 – a1 – b1) = (a1 + b1 – c1)2(2c1 – a1 – b1)。 兩個虛雙差= (a1 + b1 – c1)2(2c1 – a1 – b1) #。 所以明雙差雙差相併減於明二和共 =兩個虛雙差。 明雙差上加虛雙差即明二股共。 已知明雙差 =(b1 – c1 + a1)(c1 – a1)(2c1 – a1 – b1); 虛雙差 = (a1 + b1 – c1)2(2c1 – a1 – b1)。 明雙差上加虛雙差= (b1 – c1 + a1) (c1 – a1) (2c1 – a1 – b1) + = (a1 + b1 – c1)(2c1 – a1 – b1)[(c1 – a1) + (a1 + b1 – c1)] = (a1 + b1 – c1)(2c1 – a1 – b1) × b1 = (a1 + b1 – c1)(2c1 – a1 – b1) #。 已知日南股﹝又稱明股﹞:b14 =(c1 – a1)(b1 – c1 + a1)。 山東股﹝又稱股﹞:b15 = (c1 – b1)(a1 – c1 + b1)。 明二股共,即: (c1 – a1)(b1 – c1 + a1) + (c1 – b1)(a1 – c1 + b1) = (a1 + b1 – c1)[(c1 – a1) + (c1 – b1)] = (a1 + b1 – c1)(2c1 – a1 – b1) #。 所以明雙差上加虛雙差 = 明二股共。 雙差上加虛雙差即明二勾共也。 已知雙差 = (c1 – b1)(b1 + a1 – c1)(2c1 – a1 – b1); 虛雙差 = (a1 + b1 – c1)2(2c1 – a1 – b1)。 雙差上加虛雙差,即: (c1 – b1)(b1 + a1 – c1)(2c1 – a1 – b1) +(a1 + b1 – c1)2(2c1 – a1 – b1) = (b1 + a1 – c1)(2c1 – a1 – b1)[(c1 – b1) + (a1 + b1 – c1)] = (b1 + a1 – c1)(2c1 – a1 – b1) × a1 = (b1 + a1 – c1)(2c1 – a1 – b1) #。 已知南月勾﹝又稱明勾﹞:a14 = (c1 – a1)(b1 – c1 + a1)。 東川勾﹝又稱勾﹞:a15 =(c1 – b1)(a1 – c1 + b1)。 明二勾共 =a14 + a15 = (c1 – a1)(b1 – c1 + a1) + (c1 – b1)(a1 – c1 + b1) = (b1 – c1 + a1)[(c1 – a1) + (c1 – b1)] = (b1 + a1 – c1)(2c1 – a1 – b1) #。 所以雙差上加虛雙差 = 明二勾共。 以下為《測圓海鏡細草》原文: |
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