当n=1时，a1=s1=1²+2x1=3 当n≥2时， an=Sn-S(n-1) =n²+2n-[(n-1)²+2(n-1)] =n²+2n-(n²-2n+1+2n-2) =n²+2n-(n²-1) =2n+1 当n=1时，满足an=2n+1 则数列的通项公式an=2n+1
