A1q^4-a1q²=8⇒a1q²(q²-1)=8⇒a1q²=8/(q²-1)
a1q^8-a1q²=728⇒a1q²(q^6-1)=728⇒a1q²=728/(q^6-1)
8/(q²-1)=728/(q²-1)(q^4+q²+1)⇒q^4+q²+1=91
q^4+q²-90=0
q²=b
b²+b-90=0
b1+b2=-1 U b1*b2=-90
b1=-10⇒q²=-10 не удов усл
b1=9⇒q²=9⇒q=-3 U q=3
a1*9*(9-1)=8⇒a1=1/9
a7=a1*q^6=1/9*729=81