a1+a1*q+a1*q^2+a1*q^3=a1*(1+q^2)+a1*q(1+q^2)=a1*(1+q)(1+q^2)=240...(1)
a1*q+a1*q^3=a1*q(1+q^2)=180...(2)
两式相除,得
(1+q)/q=240/180
1/q=1/3
q=3
代入(2)
a1*3+(1+9)=180
a1=6