b1(1-q^9)/(1-q)
b1(1-q^18)/(1-q)-b1(1-q^9)/(1-q)=b1(1-q^18+q^9-1)/(1-q)
(1-q^9)/(q^9-q^18)=512
(1-q^9)/q^9(1-q^9)=512
q=1/2
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b1(1-q^9)/(1-q)
b1(1-q^18)/(1-q)-b1(1-q^9)/(1-q)=b1(1-q^18+q^9-1)/(1-q)
(1-q^9)/(q^9-q^18)=512
(1-q^9)/q^9(1-q^9)=512
q=1/2