a1+a2=a1(1+q)=3/8a3+a4+a5+a6=a1q^2(1+q)+a1q^4(1+q)=(3/8)q^2+(3/8)q^4=15/2q^4+q^2=20(q^2+5)(q^2-4)=0q^2=4 q=2a1=1/8a7=a1*q^6=(1/8)*2^6=8a7+a8+a9=a7(1+q+q^2)=8*(1+2+4)=56