12c - 15c + 19c = (12 - 15 + 19)c = 16c
14m + 21n - 21m + 8n = (14m - 21m) + (21n + 8n) = -7m + 29n = 29n - 7m
32x - (15y + 7x) + 10y = 32x - 15y - 7x + 10y = (32x - 7x) + (-15y + 10y) = 25x - 5y = 25x - 5y
8(6k + 3y) – 15(y - 6k) = 48k + 24y - 15y + 90k = (48k + 90k) + (24y - 15y) = 138k + 9y = 138k + 9y
\[ \frac{8}{9}(\frac{9}{2}x + 9) - \frac{1}{3}(9x + 24) = \frac{8}{9} \cdot \frac{9}{2}x + \frac{8}{9} \cdot 9 - \frac{1}{3} \cdot 9x - \frac{1}{3} \cdot 24 = 4x + 8 - 3x - 8 = (4x - 3x) + (8 - 8) = x + 0 = x \]
Ответ: 1) 16c; 2) 29n - 7m; 3) 25x - 5y; 4) 138k + 9y; 5) x