问题
填空题
若等比数列{an}满足a2+a4=20,a3+a5=40,则公比q=______;前n项和Sn=______.
答案
设等比数列{an}的公比为q,
∵a2+a4=20,a3+a5=40,∴
,解得a1q+a1q3=20 a1q2+a1q4=40
.a1=2 q=2
∴Sn=
=a1(qn-1) q-1
=2n+1-2.2×(2n-1) 2-1
故答案分别为2,2n+1-2.
若等比数列{an}满足a2+a4=20,a3+a5=40,则公比q=______;前n项和Sn=______.
设等比数列{an}的公比为q,
∵a2+a4=20,a3+a5=40,∴
,解得a1q+a1q3=20 a1q2+a1q4=40
.a1=2 q=2
∴Sn=
=a1(qn-1) q-1
=2n+1-2.2×(2n-1) 2-1
故答案分别为2,2n+1-2.