Question

等比数列前n项和 求详解

Answer 1

若公比q=1,则S4/S2=2≠3,因此q≠1

S4/S2=[a1(q^4 -1)/(q-1)]/[a1(q^2 -1)/(q-1)]

=(q^2+1)(q^2-1)/(q^2-1)

=q^2+1=3

q^2=2

2a2-a4

=2a2-a2q^2

=2a2-2a2

=0

选A