2. Упростите выражение: а) 6 + 4a - 5a + a - 7a; б) 5(n - 2) - 6(n + 3) - 3(2n - 9); в) 5/7 * (2,8c - 1/5d) - 2,4 * (5/6c - 1,5d)

2. Упрощаем выражения:

• а)
  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) = 5n - 10 - 6n - 18 - 6n + 27 = (5 - 6 - 6)n + (-10 - 18 + 27) = -7n + (-1) = -7n - 1.
• в)
  \[ \frac{5}{7} \left(2,8c - \frac{1}{5}d\right) - 2,4 \left(\frac{5}{6}c - 1,5d\right) = \left(\frac{5}{7} \cdot 2,8c - \frac{5}{7} \cdot \frac{1}{5}d\right) - \left(2,4 \cdot \frac{5}{6}c - 2,4 \cdot 1,5d\right) \]
  \[ = \left(\frac{14}{7}c - \frac{1}{7}d\right) - \left(\frac{12}{6}c - 3,6d\right) = (2c - \frac{1}{7}d) - (2c - 3,6d) \]
  \[ = 2c - \frac{1}{7}d - 2c + 3,6d = (2-2)c + \left(3,6 - \frac{1}{7}\right)d = \left(\frac{36}{10} - \frac{1}{7}\right)d = \left(\frac{18}{5} - \frac{1}{7}\right)d \]
  \[ = \left(\frac{18 \cdot 7 - 1 \cdot 5}{35}\right)d = \left(\frac{126 - 5}{35}\right)d = \frac{121}{35}d \]