Решение:
а) $\frac{8}{9} + \frac{5}{9} = \frac{8 + 5}{9} = \frac{13}{9}$;
б) $\frac{17}{25} - \frac{8}{25} = \frac{17 - 8}{25} = \frac{9}{25}$;
в) $\frac{31}{32} + \frac{63}{64} = \frac{62}{64} + \frac{63}{64} = \frac{62 + 63}{64} = \frac{125}{64}$;
г) $\frac{23}{68} - \frac{5}{17} = \frac{23}{68} - \frac{20}{68} = \frac{23 - 20}{68} = \frac{3}{68}$;
д) $\frac{50}{49} + \frac{15}{56} = \frac{400}{392} + \frac{105}{392} = \frac{400 + 105}{392} = \frac{505}{392}$.
Ответ:
а) $\frac{13}{9}$;
б) $\frac{9}{25}$;
в) $\frac{125}{64}$;
г) $\frac{3}{68}$;
д) $\frac{505}{392}$.