✔ 最佳答案
這不算速解法, 主要是找出規律, 化成較簡單的形式, 項數越多, 省時越多, 但硬算出純分數可就反而複雜化了
1/(2*5)+3/(5*8)+5/(8*11)+7/(11*14)+9/(14*17)+11/(17*20)
=(1/3)[(1/2-1/5)+3(1/5-1/8)+5(1/8-1/11)+7(1/11-1/14)
+9(1/14-1/17)+11(1/17-1/20)]
=(1/3)[(1/2-1/20)+2(1/5-1/8)+4(1/8-1/11)+6(1/11-1/14)
+8(1/14-1/17)+10(1/17-1/20)]
=(1/3)[(1/2-1/20)+2(1/5-1/20)+2(1/8-1/11)+4(1/11-1/14)
+6(1/14-1/17)+8(1/17-1/20)]
=(1/3)[(1/2-1/20)+2(1/5-1/20)+2(1/8-1/20)+2(1/11-1/14)
+4(1/14-1/17)+6(1/17-1/20)]
=(1/3)[(1/2-1/20)+2(1/5-1/20)+2(1/8-1/20)+2(1/11-1/20)
+2(1/14-1/17)+4(1/17-1/20)]
=(1/3)[(1/2-1/20)+2(1/5-1/20)+2(1/8-1/20)+2(1/11-1/20)
+2(1/14-1/20)+2(1/17-1/20)]
=(1/3)[(1/2+2/5+2/8+2/11+2/14+2/17)-11/20]