9. \(\frac{11}{28}+\frac{9}{35}\)

НОК(28, 35) = 140. \(\frac{11}{28}+\frac{9}{35}=\frac{55}{140}+\frac{36}{140}=\frac{91}{140}=\frac{13}{20}\).

Ответ: \(\frac{13}{20}\).