18) $$\frac{5}{6}a + \frac{7}{8}b + \frac{7}{12}a - \frac{5}{12}b =$$
$$\frac{5}{6}a + \frac{7}{12}a + \frac{7}{8}b - \frac{5}{12}b =$$
$$\frac{10}{12}a + \frac{7}{12}a + \frac{21}{24}b - \frac{10}{24}b =$$
$$\frac{17}{12}a + \frac{11}{24}b =$$
$$1\frac{5}{12}a + \frac{11}{24}b$$
Ответ: $$1\frac{5}{12}a + \frac{11}{24}b$$