an=-(n^2+2n-1)/(n+3)
原式=[-(1^2+2*1-1)/(1+3)]*[-(2^2+2*2-1)/(2+3]*[-(3^2+2*3-1)/(3+3)]*...*[-(8^2+2*8-1)/(1+8)]
=5303+38711/59400
没有什么简便的算法,只好用excel来算了.