а) Ѕ при а = 5 и в=;

a) $$S = a \cdot b$$

$$a = 5\frac{1}{7} = \frac{5 \cdot 7 + 1}{7} = \frac{36}{7}$$

$$b = \frac{4}{9}$$

$$S = \frac{36}{7} \cdot \frac{4}{9} = \frac{36 \cdot 4}{7 \cdot 9} = \frac{4 \cdot 9 \cdot 4}{7 \cdot 9} = \frac{16}{7} = 2\frac{2}{7}$$

Ответ: $$2\frac{2}{7}$$