Question

设数列{an}的前n项和为Sn,a1=1,an+1=4sn,求a5的值.

Answer 1

A1=1 S1=1

A2=4*S1=4 S2=5

A3=4*S2=20 S3=25

A4=4*s3=100 s4=125

A5=4*s4=600