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

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1/4+1/12+1/24+1/40.+1/198001/4+1/12+1/24+1/40.+1/198001/4+1/12+1/24+1/40.+1/19800An=1/2n(n+1)=0.5*((

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

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