\[\boxed{\text{452\ (452).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
Пояснение.
Решение.
\[1)\ \left\{ a,\ b \right\} \cap \left\{ a \right\} = a;\]
\[2)\ \left\{ a,\ b \right\} \cap \left\{ a \right\} \neq \left\{ a,b \right\};\]
\[3)\ \left\{ a,b \right\} \cap \left\{ a \right\} = \left\{ a \right\};\]
\[4)\ \left\{ a,b \right\} \cap \left\{ a \right\} \neq \left\{ b \right\}\text{.\ }\]