a1=s1 = 3+1-1 = 3an = sn -sn-1 = 3n² +n-1 - (3(n-1)² +(n-1) -1)= 3(n+n-1)*1 +1= 3(2n -1) +1= 6n -2n=1时a1 =3 n>=2时 an = 6n -2
an=sn-s(n-1)
