5.488 Найдите значение выражения: 3 -a 3 а) =а при а == 7 5 12 при 6 = 1, 5' 5

a)

При \(a = \frac{3}{7}\):

\[\frac{3}{7}a = \frac{3}{7} \cdot \frac{3}{7} = \frac{3 \cdot 3}{7 \cdot 7} = \frac{9}{49}\]

При \(a = \frac{119}{66}\):

\[\frac{3}{7}a = \frac{3}{7} \cdot \frac{119}{66} = \frac{3 \cdot 119}{7 \cdot 66} = \frac{357}{462}\]

Сокращаем дробь на 21:

\[\frac{357:21}{462:21} = \frac{17}{22}\]

При \(a = \frac{28}{33}\):

\[\frac{3}{7}a = \frac{3}{7} \cdot \frac{28}{33} = \frac{3 \cdot 28}{7 \cdot 33} = \frac{84}{231}\]

Сокращаем дробь на 21:

\[\frac{84:21}{231:21} = \frac{4}{11}\]

б)

При \(b = \frac{1}{5}\):

\[\frac{5}{12}b = \frac{5}{12} \cdot \frac{1}{5} = \frac{5 \cdot 1}{12 \cdot 5} = \frac{5}{60}\]

Сокращаем дробь на 5:

\[\frac{5:5}{60:5} = \frac{1}{12}\]

При \(b = \frac{5}{12}\):

\[\frac{5}{12}b = \frac{5}{12} \cdot \frac{5}{12} = \frac{5 \cdot 5}{12 \cdot 12} = \frac{25}{144}\]

При \(b = \frac{6}{5}\):

\[\frac{5}{12}b = \frac{5}{12} \cdot \frac{6}{5} = \frac{5 \cdot 6}{12 \cdot 5} = \frac{30}{60}\]

Сокращаем дробь на 30:

\[\frac{30:30}{60:30} = \frac{1}{2}\]