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

Решение:

1. а) 6 + 4a – 5a + a – 7a = 6 + (4 – 5 + 1 – 7)a = 6 – 7a
2. 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
3. в) 5/7 · (2,8c – 4 1/5d) – 2,4 · (5/6c – 1,5d)
• = 5/7 · (28/10c – 21/5d) – 24/10 · (5/6c – 3/2d)
• = 5/7 · (14/5c – 21/5d) – 12/5 · (5/6c – 3/2d)
• = (5/7 · 14/5c) – (5/7 · 21/5d) – (12/5 · 5/6c) + (12/5 · 3/2d)
• = 2c – 3d – 2c + 18/5d
• = (2c – 2c) + (–3d + 18/5d)
• = 0c + (–15/5d + 18/5d) = 3/5d = 0,6d

Ответ: а) 6 – 7a; б) –7n – 1; в) 0,6d