konuyu yeni gördüm
ben de tam hesaplamıştım :P :P

http://www.wolframalpha.com/input/?i...+to+infinite++[%28%28n*%28n-1%29*%28n-2%29%29%2F+3^%28n-3%29%29]

edit: yeni link: http://tinyurl.com/ntln376