$rac{4}{9} + rac{5}{9} = rac{4+5}{9} = rac{9}{9} = 1$

$rac{5}{11} + rac{5}{11} = rac{5+5}{11} = rac{10}{11}$

$rac{2}{7} + rac{3}{7} = rac{2+3}{7} = rac{5}{7}$

$rac{7}{15} - rac{13}{15} = rac{7-13}{15} = rac{6}{15} = rac{2}{5}$

$rac{10}{13} - rac{2}{13} = rac{2-10}{13} = rac{8}{13}$

$rac{8}{10} - rac{8}{10} = rac{8-8}{10} = rac{0}{10} = 0$