1/4+1/12+1/24+1/40.+1/19800

问题描述:

1/4+1/12+1/24+1/40.+1/19800

An=1/2n(n+1)=0.5*((1/n)-(1/n+1))
Sn=0.5*(1-1/n+1)
An=1/19800得n=99
Sn=99/200