等比数列{an}中,由3+n-2=1+n
得a3·a(n-2)=a1*an=27
又a1+an=28
解得a1=1,an=27或a1=27,an=1
1、当a1=1,an=27时
sn＝(a1-q*an)/(1-q)=40,即(1-27q)/(1-q)=40,q=3
此时,an=a1*q^(n-1),即27=3^(n-1),n=4
2、当a1=27,an=1时
(27-q)/(1-q)=40,q＝1/3,同样n=4