5775 18) --а+в+a= 6812 12

18) $$\frac{5}{6}a + \frac{7}{8}b + \frac{7}{12}a - \frac{5}{12}b = (\frac{5}{6} + \frac{7}{12})a + (\frac{7}{8} - \frac{5}{12})b = (\frac{10}{12} + \frac{7}{12})a + (\frac{21}{24} - \frac{10}{24})b = \frac{17}{12}a + \frac{11}{24}b$$

Ответ: $$\frac{17}{12}a + \frac{11}{24}b$$