Sn=2n^2+4n
S(n-1)=2(n-1)^2+4(n-1)
an=Sn-S(n-1)
=2n^2+4n-2(n-1)^2-4(n-1)
=2n^2+4n-2n^2+4n-2-4n+4
=4n+2
所以 an=4n+2