设公比为q,则q>0
a3=a2+4
a1q^2=a1q+4
a1=2代入,整理,得
q^2-q-2=0
(q+1)(q-2)=0
q=-1(舍去)或q=2
Sn=a1(q^n -1)/(q-1)=2×(2^n -1)/(2-1)=2^(n+1) -2