5.409 Найдите сумму или разность: a) 1/5 + 1/7; б) 1/4 + 1/9; в) 4/7 + 4/9; г) 1/2 + 5/7; д) 9/11 + 0; е) 3/4 - 3/5;

a) $$ \frac{1}{5} + \frac{1}{7} = \frac{7 + 5}{35} = \frac{12}{35} $$ б) $$ \frac{1}{4} + \frac{1}{9} = \frac{9 + 4}{36} = \frac{13}{36} $$ в) $$ \frac{4}{7} + \frac{4}{9} = \frac{4 \cdot 9 + 4 \cdot 7}{63} = \frac{36 + 28}{63} = \frac{64}{63} = 1 \frac{1}{63} $$ г) $$ \frac{1}{2} + \frac{5}{7} = \frac{7 + 10}{14} = \frac{17}{14} = 1 \frac{3}{14} $$ д) $$ \frac{9}{11} + 0 = \frac{9}{11} $$ е) $$ \frac{3}{4} - \frac{3}{5} = \frac{3 \cdot 5 - 3 \cdot 4}{20} = \frac{15 - 12}{20} = \frac{3}{20} $$