2/1+6/5+12/11+20/19+.90/89
问题描述：
2/1+6/5+12/11+20/19+.90/89
答
1/2+5/6+11/12+19/20+…………＋89/90=(1-1/2)+(1-1/6)+(1-1/12)+(1-1/20)+……+(1-1/90)=9-(1/2+1/6+1/12+1/20……+1/90) =9-[1/2+(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+(1/5-1/6)+……+(1/9-1/10)]=9-[1/2+1/2-1/3+1/3-1...