第五题,高中等比数列,

S2=a1（1+q）=7
S4=S2+S2q^2
S6=S2+S2q^2+S2q^4=91
q^4+q^2-12=0
q=根号3
S4=28
S6=S2+S2q^2+S2q^4=91
S2=7
带入上式,7+7q^2+7q^4=91
1+q^2+q^4=13
移项
q^4+q^2-12=0
or:
s2=a1(1-q^2)/(1-q)=7 ①
s6=a1(1-q^6)/(1-q)=91 ②
②/①,得q^4+q^2-12=0,q^2=3代入①得,
a1/(1-q)=7/(1-3)=-7/2
s4=a1(1-q4)/(1-q)=(-7/2)*(1-3^2)=28