\[\boxed{\mathbf{421\ }\left( \mathbf{421} \right)\mathbf{.}\mathbf{\ }Еуроки\ - \ ДЗ\ без\ мороки}\]
Пояснение.
Решение.
\[a \neq 0;\ \ \ b \neq 0\]
\[1)\ \frac{3}{4}a = \frac{2}{3}\text{b\ \ \ \ }\]
\[a = \frac{2}{3}b\ :\frac{3}{4}\]
\[a = \frac{2}{3} \cdot \frac{4}{3} \cdot b\]
\[a = \frac{2 \cdot 4}{3 \cdot 3}\text{b\ \ \ }\]
\[a = \frac{8}{9}b\ \Longrightarrow a < b.\]
\[2)\ \frac{2}{5}a = \frac{5}{7}\text{b\ \ \ \ }\]
\[a = \frac{5}{7}b\ :\frac{2}{5}\]
\[a = \frac{5}{7} \cdot \frac{5}{2} \cdot b\]
\[a = \frac{5 \cdot 5}{7 \cdot 2}\text{b\ \ \ \ }\]
\[a = \frac{25}{14}b\ \Longrightarrow a > b.\]