要注意a[0]=3.起始值是不一样的.通项不是a[n]=n+1.
提问楼主：您确定这题没抄错么?用Mathematica计算出前N项的和(N=1..7)分别为：
1/10,
101/910,
81901/737100,
5371535234701/48343220207100,
11552786200945219795469238149/103973792019429615826920822900,
290827857090024178581170863000160221506001885743471327881/2617418395924312792436366140596420162678683490959811446100,
24891394287555229879294017965720435653582680251610016640910152075330940854377398469301711873276182413278935248165583079/224019782562594117368036515001000751238304813279357894671873108104760331661626103931210338076738704278061954610751327400
这些已经是最简分数.看起来不像能用一个简单的式子就能表达啊.