若数列{an}满足:a1=1,an+1=2an(n∈N*),则a5=_;前8项的和S8=_.(用数字作答)
问题描述:
若数列{an}满足:a1=1,an+1=2an(n∈N*),则a5=______;前8项的和S8=______.(用数字作答)
答
a1=1,a2=2a1=2,a3=2a24,a4=2a3=8,a5=2a4=16,
∵an+1=2an,即
=2an+1 an
∴数列{an}为等比数列,首项为1,公比为2.
∴S8=
=255,
28−1 2−1
∴故答案为:16,255.