1/(x方+3x)=(1/3)*(1/x-1/(x+3))。将x=1,2,……,n代入Sn,得:Sn=(1/3)*(1-1/4+1/2-1/5+1/3-1/6+……+1/n-1/3n)当n趋近于无穷时,Sn=(1/3)*(1+1/2+1/3)=11/18
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