等比数列｛an}中,a3=4,s3=12,求q
问题描述：
等比数列｛an}中,a3=4,s3=12,求q
答
s3=a1+a2+a3
=a3/q^2+a3/q+a3
=a3(1+1/q+1/q^2)
12=4(1+1/q+1/q^2)
1+1/q+1/q^2=3
1/q+1/q^2=2
q+1=2q^2
2q^2-q-1=0
(2q+1)(q-1)=0
q=-1/2或q=1