а) \[ \frac{5}{11} + \frac{4}{11} = \frac{5+4}{11} = \frac{9}{11} \]
б) \[ \frac{6}{17} + \frac{3}{17} = \frac{6+3}{17} = \frac{9}{17} \]
в) \[ \frac{12}{19} - \frac{4}{19} = \frac{12-4}{19} = \frac{8}{19} \]
г) \[ \frac{8}{25} - \frac{3}{25} = \frac{8-3}{25} = \frac{5}{25} \]
Ответ: a) \(\frac{9}{11}\); б) \(\frac{9}{17}\); в) \(\frac{8}{19}\); г) \(\frac{5}{25}\)