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
S4²=S2×S7=7×91=637，S4=√637
由s2,s4-s2,s6-s4是等比数列，得出
