Решение:
а) $\frac{-1}{3} + \frac{2}{3} = \frac{+(2 - 1)}{3} = \frac{1}{3};$
б) $\frac{-1}{4} + \frac{3}{4} = \frac{+(3 - 1)}{4} = \frac{2}{4} = \frac{1}{2};$
в) $\frac{1}{5} + \frac{-3}{5} = \frac{-(3 - 1)}{5} = \frac{-2}{5} = -\frac{2}{5};$
г) $\frac{3}{7} + \frac{-4}{7} = \frac{-(4 - 3)}{7} = \frac{-1}{7} = -\frac{1}{7};$
д) $\frac{8}{13} + \frac{-12}{13} = \frac{-(12 - 8)}{13} = \frac{-4}{13} = -\frac{4}{13};$
е) $\frac{19}{25} + \frac{-24}{25} = \frac{-(24 - 19)}{25} = \frac{-5}{25} = -\frac{1}{5}.$
Ответ:
а) $\frac{1}{3};$
б) $\frac{1}{2};$
в) $-\frac{2}{5};$
г) $-\frac{1}{7};$
д) $-\frac{4}{13};$
е) $-\frac{1}{5}.$