Sn=na1+n(n-1)d/2
an=a1-d+nd
limSn/(an)^2=lim(na1+n(n-1)d/2)/(a1-d+nd)^2(n->无穷)
=1/2d=1/6
d=3
an是等差数列,前n项和是Sn,若limSn/(an)^2=1/6,求公差d.
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