[tex]\it \dfrac{b_1+b_3+b_5+ ... +b_{2n-1}}{b_2+b_4+b_6+...+b_{2n}}=3 \Longrightarrow \dfrac{b_1(1+q^2+q^4+ ... + q^{2n-2})}{b_1(q+q^3+q^5+ ... +q^{2n-1})} =3
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\Longrightarrow \dfrac{1+q^2+q^4+ ... + q^{2n-2}}{q(1+q^2+q^4+ ... + q^{2n-2} )} =3 \Longrightarrow \dfrac{1}{q} = 3 \Longrightarrow q= \dfrac{1}{3}[/tex]
