\[\boxed{\text{446\ (446).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
Пояснение.
Решение.
\[1)\ A = \left\{ 2,\ 7,\ 8 \right\};\ \ B = \{ 3,\ 5,\ 6,\ 8\}\]
\[\ \ A \cup B = \left\{ 2,\ 3,\ 5,\ 6,\ 7,\ 8 \right\};\]
\[2)\ A = \left\{ 5 \right\},;\ B = \{ 7\}\]
\[\ \ A \cup B = \left\{ 5,\ 7 \right\}.\ \]