a2＝1/4,s3＝7a3,求公比q

更新时间2018-04-22 17:57:22

在等比数列中

设首项为a1

a2=a1q

a3=a1q^2

S3=a1+a2+a3=a1+a1q+a1q^2

a1+a1q+a1q^2=7a1q^2

1+q+q^2=7q^2

6q^2-q-1=0

(2q-1)(3q+1)=0

2q-1=0,    q=1/2

3q+1=0,   q=-1/3