[tex]b_3=b_1*q^2 [/tex]
[tex]b_5=b_1*q^4 [/tex]
[tex]\left \{ {{b_1+b_1*q^2=5} \atop {b1*q^2+b_1*q^4=20}} \right. [/tex]
[tex]\left \{ {{b_1(1+q^2)=5} \atop {b_1*q^2(1+q^2)=20}} \right.\Rightarrow
\frac{b_1(1+q^2)}{b_1*q^2(1+q^2)} =\frac{5}{20} [/tex]
[tex]\frac{\not{b}_1}{\not{b_1}*q^2}=\frac{1}{4} \rightarrow \frac{1}{q^2}= \frac{1}{4} [/tex]
[tex]q^2=4 \Rightarrow q=\pm2 [/tex]
