\[\boxed{\text{626\ (}\text{с}\text{).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ b_{1} = \frac{3}{4};\ \ q = \frac{2}{3}:\]
\[b_{5} = b_{1} \cdot q^{4} = \frac{3}{4} \cdot \left( \frac{2}{3} \right)^{4} = \frac{3}{2^{2}} \cdot \frac{2^{4}}{3^{4}} = \frac{2^{2}}{3^{3}} = \frac{4}{27}.\]
\[\textbf{б)}\ b_{1} = 1,8;\ \ \ q = \frac{\sqrt{3}}{3}:\]
\[b_{4} = b_{1} \cdot q^{3} = 1,8 \cdot \left( \frac{\sqrt{3}}{3} \right)^{3} = 1,8 \cdot \left( \frac{1}{\sqrt{3}} \right)^{3} = \frac{1,8}{3} \cdot \frac{1}{\sqrt{3}} = \frac{0,6}{\sqrt{3}} = \frac{\sqrt{3}}{5}.\]