Выполните действия, применив правила сложения и вычитания дробей с разными знаменателями.

Чтобы сложить или вычесть дроби с разными знаменателями, нужно привести их к общему знаменателю (найти НОК). **а)** 1) $\frac{1}{3} + \frac{2}{7} = \frac{1 \cdot 7}{21} + \frac{2 \cdot 3}{21} = \frac{7 + 6}{21} = \frac{13}{21}$ 2) $\frac{7}{10} - \frac{3}{5} = \frac{7}{10} - \frac{6}{10} = \frac{1}{10}$ 3) $\frac{3}{8} + \frac{9}{20} = \frac{3 \cdot 5}{40} + \frac{9 \cdot 2}{40} = \frac{15 + 18}{40} = \frac{33}{40}$ **б)** 1) $\frac{4}{9} + \frac{1}{5} = \frac{20}{45} + \frac{9}{45} = \frac{29}{45}$ 2) $\frac{5}{6} - \frac{1}{2} = \frac{5}{6} - \frac{3}{6} = \frac{2}{6} = \frac{1}{3}$ 3) $\frac{11}{20} + \frac{7}{15} = \frac{33}{60} + \frac{28}{60} = \frac{61}{60} = 1 \frac{1}{60}$ **в)** 1) $\frac{2}{3} - \frac{2}{5} = \frac{10}{15} - \frac{6}{15} = \frac{4}{15}$ 2) $\frac{1}{2} + \frac{5}{8} = \frac{4}{8} + \frac{5}{8} = \frac{9}{8} = 1 \frac{1}{8}$ 3) $\frac{5}{9} - \frac{5}{12} = \frac{20}{36} - \frac{15}{36} = \frac{5}{36}$ **г)** 1) $\frac{3}{5} - \frac{4}{7} = \frac{21}{35} - \frac{20}{35} = \frac{1}{35}$ 2) $\frac{4}{9} + \frac{4}{27} = \frac{12}{27} + \frac{4}{27} = \frac{16}{27}$ 3) $\frac{7}{12} - \frac{3}{20} = \frac{35}{60} - \frac{9}{60} = \frac{26}{60} = \frac{13}{30}$ **д)** 1) $\frac{5}{9} + \frac{2}{7} = \frac{35}{63} + \frac{18}{63} = \frac{53}{63}$ 2) $\frac{6}{7} - \frac{14}{49} = \frac{42}{49} - \frac{14}{49} = \frac{28}{49} = \frac{4}{7}$ 3) $\frac{5}{6} - \frac{8}{15} = \frac{25}{30} - \frac{16}{30} = \frac{9}{30} = \frac{3}{10}$ **е)** 1) $\frac{5}{6} + \frac{4}{5} = \frac{25}{30} + \frac{24}{30} = \frac{49}{30} = 1 \frac{19}{30}$ 2) $\frac{19}{60} - \frac{3}{20} = \frac{19}{60} - \frac{9}{60} = \frac{10}{60} = \frac{1}{6}$ 3) $\frac{13}{15} - \frac{13}{20} = \frac{52}{60} - \frac{39}{60} = \frac{13}{60}$ **ж)** 1) $\frac{5}{7} + \frac{7}{9} = \frac{45}{63} + \frac{49}{63} = \frac{94}{63} = 1 \frac{31}{63}$ 2) $\frac{29}{60} - \frac{7}{30} = \frac{29}{60} - \frac{14}{60} = \frac{15}{60} = \frac{1}{4}$ 3) $\frac{5}{42} + \frac{10}{63} = \frac{15}{126} + \frac{20}{126} = \frac{35}{126} = \frac{5}{18}$ **з)** 1) $\frac{3}{4} + \frac{5}{9} = \frac{27}{36} + \frac{20}{36} = \frac{47}{36} = 1 \frac{11}{36}$ 2) $\frac{7}{8} - \frac{5}{24} = \frac{21}{24} - \frac{5}{24} = \frac{16}{24} = \frac{2}{3}$ 3) $\frac{7}{24} + \frac{11}{40} = \frac{35}{120} + \frac{33}{120} = \frac{68}{120} = \frac{17}{30}$