根据条件an=x^(n-1)Sn=(1-x^n)/(1-x)故 f(x)=lim(n->无穷)[2Sn/(1+Sn)] =lim(n->无穷)[2/(1+1/Sn)] =lim(n->.无穷)[2/(1+(1-x)/(1-x^n)] 若 x<=1,则 f(x)=2/(2-x) 若 x>1, 则 f(x)=2
