Выразим все элементы системы через b1 и q:
\left \{ {{b_1+b_1q^2=20} \atop {b_1q+b_1q^{3}=60}} \right \ => \left \{ {{b_1(1+q^2)=20} \atop {b_1(q+q^{3})=60}} \right \ =>\\\\ \left \{ {{b_1=\frac{20}{(1+q^2)}} \atop {b_1q(1+q^{2})=60}} \right \ \ =>\frac{20}{1+q^{2}}q(1+q^{2})=60=>\\\\ q=3=>b_1=\frac{20}{1+3^{2}}=10 " alt="\left \{ {{b_1+b_3=20} \atop {b_2+b_4=60}} \right \ => \left \{ {{b_1+b_1q^2=20} \atop {b_1q+b_1q^{3}=60}} \right \ => \left \{ {{b_1(1+q^2)=20} \atop {b_1(q+q^{3})=60}} \right \ =>\\\\ \left \{ {{b_1=\frac{20}{(1+q^2)}} \atop {b_1q(1+q^{2})=60}} \right \ \ =>\frac{20}{1+q^{2}}q(1+q^{2})=60=>\\\\ q=3=>b_1=\frac{20}{1+3^{2}}=10 " align="absmiddle" class="latex-formula">