Упрощение выражений

a) 6 + 4a - 5a + a - 7a = 6 + (4 - 5 + 1 - 7)a = 6 - 7a

б) 5(n - 2) - 6(n + 3) - 3(2n - 9) = 5n - 10 - 6n - 18 - 6n + 27 = (5 - 6 - 6)n + (-10 - 18 + 27) = -7n - 1

в) $$\frac{5}{7}(2,8c - \frac{4}{5}d) - 2,4(\frac{5}{6}c - 1,5d)$$ $$\frac{5}{7} \cdot 2,8c - \frac{5}{7} \cdot \frac{4}{5}d - 2,4 \cdot \frac{5}{6}c + 2,4 \cdot 1,5d =$$ $$2c - \frac{4}{7}d - 2c + 3,6d = $$ $$-\frac{4}{7}d + 3,6d = $$ $$-\frac{4}{7}d + \frac{36}{10}d = $$ $$-\frac{4}{7}d + \frac{18}{5}d = $$ $$\frac{-4 \cdot 5 + 18 \cdot 7}{35}d = $$ $$\frac{-20 + 126}{35}d = $$ $$\frac{106}{35}d = $$ $$3 \frac{1}{35}d$$