П.21 Найдите значение выражения: a) 5 : 10 - 5.2. 12 3 6 15' б) 4.5 + 7.4 25 16 16 5

а)

\[\frac{5}{12} : \frac{10}{3} - \frac{5}{6} \cdot \frac{2}{15} = \frac{5}{12} \cdot \frac{3}{10} - \frac{5}{6} \cdot \frac{2}{15} = \frac{5 \cdot 3}{12 \cdot 10} - \frac{5 \cdot 2}{6 \cdot 15} = \frac{\cancel{5} \cdot \cancel{3}}{4 \cdot \cancel{3} \cdot 2 \cdot \cancel{5}} - \frac{\cancel{5} \cdot \cancel{2}}{3 \cdot \cancel{2} \cdot 3 \cdot \cancel{5}} = \frac{1}{8} - \frac{1}{9} = \frac{9}{72} - \frac{8}{72} = \frac{9-8}{72} = \frac{1}{72}\]

б)

\[\frac{4}{25} \cdot \frac{5}{16} + \frac{7}{16} \cdot \frac{4}{5} = \frac{4 \cdot 5}{25 \cdot 16} + \frac{7 \cdot 4}{16 \cdot 5} = \frac{\cancel{4} \cdot \cancel{5}}{5 \cdot \cancel{5} \cdot 4 \cdot \cancel{4}} + \frac{7 \cdot \cancel{4}}{4 \cdot \cancel{4} \cdot 5} = \frac{1}{20} + \frac{7}{20} = \frac{1+7}{20} = \frac{8}{20} = \frac{2}{5}\]

Ответ: а) \(\frac{1}{72}\); б) \(\frac{2}{5}\)