1/(2*5)+1/(3*5)+1/(8*11)+...+1/(20*23)怎么感觉一点规律也没有啊.....

问题描述:

1/(2*5)+1/(3*5)+1/(8*11)+...+1/(20*23)
怎么感觉一点规律也没有啊.....

不得不说,此题出题出错了,应该是复印错了,
原题应该是:1/(2*5)+1/(5*8)+1/(8*11)+1/(11*14)+1/(14*17)+1/(17*20)+1/(20*23)
答案应该是(1/2-1/5+1/5-1/8+1/8-1/11+1/11-1/14+1/14-1/17+1/17-1/20+1/20-1/23)/3=(1/2-1/23)/3=(21/46)/3=7/46
答案为7/46

1/(2*5)=(1/3)(1/2-1/5)
1/(3*5)=(1/2)(1/3-1/5)
再推下去,就可以了

如果是把“1/(3*5)+”换成“1/(5*8)+”就对了2,5,8,11,14,17,20,23..顺序增加31/(2*5)+1/(5*8)+1/(8*11)+...+1/(20*23)=(1/3)*(1/2-1/5+1/5-1/8+1/8-1/11+.+1/20-1/23)=(1/3)*(1/2-1/23)=7/46

1/(2*5)=(1/3)(1/2-1/5)
1/(3*5)=(1/2)(1/3-1/5)
以此类推