∵a(n+1) = S(n+1) - Sn∴S(n+1) - Sn = 1/2Sn∴S(n+1) = 3/2Sn∴数列{Sn}是以S1 = a1 = 1为首项， 3/2为公比的对比数列。∴Sn = 1×3/2^(n-1) = 3/2^(n-1)
