Question

等比数列习题等比数列｛An｝中,S6=91 ,S2=7,

Answer 1

1.

(S4-S2)(S4-S2)=S2*(S6-S4)

S4=28

2.

S6/S2=(1-q^6)/(1-q^2)

=(1-q^2)(1+q^2+q^4)/(1-q^2)

=1+q^2+q^4=13

q^4=12-q^2

q^4+q^2-12=0

(q^2+4)(q^2-3)=0

q^2=3

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

=(1-12+q^2)/(1-q^2)

=(3-11)/(-2)

=4

S4=S2*4=28