To add ( \frac{5}{6} + \frac{3}{4} ), we first need to find a common denominator for both fractions. The least common denominator for 6 and 4 is 12.

( \frac{5}{6} + \frac{3}{4} = \frac{5 \times 2}{6 \times 2} + \frac{3 \times 3}{4 \times 3} = \frac{10}{12} + \frac{9}{12} = \frac{19}{12} ).

To subtract ( \frac{9}{10} - \frac{1}{6} ), we need to find a common denominator. The least common denominator for 10 and 6 is 30.

( \frac{9}{10} - \frac{1}{6} = \frac{9 \times 3}{10 \times 3} - \frac{1 \times 5}{6 \times 5} = \frac{27}{30} - \frac{5}{30} = \frac{22}{30} = \frac{11}{15} ).

Therefore, ( \frac{5}{6} + \frac{3}{4} = \frac{19}{12} ) and ( \frac{9}{10} - \frac{1}{6} = \frac{11}{15} ).