a1=s1=2,当n>租手枣=2时,an=Sn-S(n-1)=n^2-(n-1)^2=2n-1;当n=1时,a1=2不等于(2*1-1),所以a1不符合通项弊拆,所以薯卖an=2(n=1),an=2n-1(n>=2)