Tính: 1/6 . -9-5 . -15/7

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4A. Tính:

a) $\frac{1}{6} - \frac{9}{5} \left( - \frac{15}{7} \right)$;

b) $\frac{3}{4} \cdot \frac{5}{9} + \frac{6}{7} \cdot \frac{4}{3} - \left( \frac{1}{2} \cdot \frac{1}{3} \right)$;

c) $2 + \frac{1}{2} - \frac{4}{3} \div \frac{2}{1}$;

d) $\left( 1 - \frac{2}{3} - \frac{1}{4} \right) \cdot \left( 2 - \frac{4}{3} - \frac{4}{5} \right)$.