an=Sn-1
Sn-Sn-1=Sn-1
Sn=2Sn-1
S1=a1=5
所以,{Sn}是首项为5,公比为2的等比数列
Sn=5*(2^n-1)
Sn-1=5*(2^(n-1)-1)
an=Sn-Sn-1
=5[(2^n-1)-(2^(n-1)-1)]
=5(2^n-2^(n-1))
=5*2^(n-1)