an=Sn-1Sn-Sn-1=Sn-1Sn=2Sn-1S1=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)
