Решение:
а) $\frac{5}{8} + (-\frac{9}{8}) = \frac{5}{8} - \frac{9}{8} = \frac{5 - 9}{8} = \frac{-4}{8} = -\frac{1}{2};$
б) $-\frac{3}{13} + (-\frac{8}{13}) = \frac{-3}{13} - \frac{8}{13} = \frac{-3 - 8}{13} = \frac{-11}{13};$
в) $-\frac{2}{5} + \frac{4}{5} = \frac{-2}{5} + \frac{4}{5} = \frac{-2 + 4}{5} = \frac{2}{5};$
г) $\frac{3}{8} + (-\frac{3}{4}) = \frac{3}{8} - \frac{3}{4} = \frac{3}{8} - \frac{6}{8} = \frac{3 - 6}{8} = \frac{-3}{8} = -\frac{3}{8};$
д) $-\frac{7}{15} + (\frac{-2}{3}) = \frac{-7}{15} + \frac{-10}{15} = \frac{-7 - 10}{15} = \frac{-17}{15} = -1\frac{2}{15};$
е) $-\frac{7}{8} - \frac{15}{16} = \frac{-14}{16} - \frac{15}{16} = \frac{-14 - 15}{16} = \frac{-29}{16} = -1\frac{13}{16};$
ж) $\frac{1}{3} + (\frac{-1}{2}) = \frac{2}{6} + \frac{-3}{6} = \frac{2 - 3}{6} = \frac{-1}{6} = -\frac{1}{6};$
з) $-\frac{1}{4} + \frac{1}{3} = \frac{-3}{12} + \frac{4}{12} = \frac{-3 + 4}{12} = \frac{1}{12};$
и) $-\frac{2}{21} + \frac{3}{14} = \frac{-4}{42} + \frac{9}{42} = \frac{-4 + 9}{42} = \frac{5}{42}.$
Ответ:
а) $-\frac{1}{2};$
б) $-\frac{11}{13};$
в) $\frac{2}{5};$
г) $-\frac{3}{8};$
д) $-1\frac{2}{15};$
е) $-1\frac{13}{16};$
ж) $-\frac{1}{6};$
з) $\frac{1}{12};$
и) $\frac{5}{42}.$