注：2/n(n+2)=1/n-1/(n+2)
感觉题目好像错了,理论上原题应该是
20-(2|3+2|15+2|35+2|63+2|99)
=20-(1-1/3+1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11）
=20-(1-1/11)
=20-10/11
=210/11
类似该题的有1/2+1/6+1/12+...+1/n(n+1)=n/(n+1)
1/n(n+1)=1/n-1/(n+1)