Sn=n平方+2n
S(n-1)=(n-1)²+2(n-1)
an=Sn-S(n-1)
=[n²-(n-1)²]+[2n-2(n-1)]
=(n+n-1)(n-n+1)+2(n-n+1)
=2n-1+2
=2n+1