救急，数学题

an=a1+(n-1)d=3+(n-1)*2=2n+1Sn=(a1+an)n/2=(3+2n+1)n/2=n(n+2)

Sn=a1*n+1/2(n-1)*n*d =3n+n^2-n =n^2+2n (n^2是n的二次方）