6) 6c - 8c - 4c + 21 - 13; б) 1/4a + 5/6b - 1/8a - 1/3b;

6)

• \[ 6c - 8c - 4c + 21 - 13 = (6-8-4)c + (21-13) = -6c + 8 \]

б)

• \[ \frac{1}{4}a + \frac{5}{6}b - \frac{1}{8}a - \frac{1}{3}b = \left( \frac{1}{4} - \frac{1}{8} \right)a + \left( \frac{5}{6} - \frac{1}{3} \right)b = \left( \frac{2}{8} - \frac{1}{8} \right)a + \left( \frac{5}{6} - \frac{2}{6} \right)b = \frac{1}{8}a + \frac{3}{6}b = \frac{1}{8}a + \frac{1}{2}b \]

Ответ: 6) -6c + 8; б) 1/8a + 1/2b