18)--а+в+а-б =

$$18) -\frac{5}{6}a + \frac{7}{8}b + \frac{7}{12}a - \frac{5}{12}b = (-\frac{5}{6} + \frac{7}{12})a + (\frac{7}{8} - \frac{5}{12})b = (\frac{-10 + 7}{12})a + (\frac{21 - 10}{24})b = -\frac{3}{12}a + \frac{11}{24}b = -\frac{1}{4}a + \frac{11}{24}b$$

Ответ: $$- \frac{1}{4}a + \frac{11}{24}b$$