Sn＝n^2an ,当n>=2时,Sn-1＝（n-1）^2an-1
两式子相减得an=n^2an-（n-1）^2an-1,化简得an/an-1=（n-1）/（n+1）
再用迭乘法求通项就可以了
a1＝1 Sn＝n^2an 求an
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