前三项是a、4、3a,则a+3a=2×4=8,得:a=2,则:an=2n,Sn=n(n+1).1/(Sn)=1/n-1/(n+1),则所求的和=[(1/1)-(1/2)]+[(1/2)-(1/3)]+…[(1/2009)-(1/2010)]=2009/2010
已知等差数列前三项韦a.4.3a.前n项和为Sn,又Sk=2550.求1/S1+1/S2+…+1/S2010
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