设等差为p
an=a1+(n-1)p
Sn=(a1+a1+(n-1)p)*n/2
lim(n*an/Sn)
=lim(n*(a1+(n-1)p)/((a1+a1+(n-1)p)*n/2)
=lim(2*(a1+(n-1)p)/(2a1+(n-1)p))
n趋向无穷时,n-1>>a1
所以等式趋向2