Question

等比数列{an}中,已知a1+a2=20,a3+a4=40,则a5+a6=

Answer 1

a1+a2=20,

a3+a4=q^2(a1+a2)=40

q^2=2

a5+a6=q^2(a3+a4)=2*40=80

a1+a2=3,

a2+a3=q(a1+a2)=6

q=2

a1+a1*2=3

a1=1

a7=a1q^6=2^6=64