4) Выполните сложение (вычитание) дробей с разными знаменателями: а) 8/15 + 7/12; 21/50 − 7/30; 5/42 + 5/36; б) 11/24 + 4/15; 11/30 − 7/80; 19/35 + 10/21.

а)

1. \(\frac{8}{15}+\frac{7}{12}=\frac{32}{60}+\frac{35}{60}=\frac{67}{60}=1\frac{7}{60}\).
2. \(\frac{21}{50}-\frac{7}{30}=\frac{63}{150}-\frac{35}{150}=\frac{28}{150}=\frac{14}{75}\).
3. \(\frac{5}{42}+\frac{5}{36}=\frac{30}{252}+\frac{35}{252}=\frac{65}{252}\).

б)

1. \(\frac{11}{24}+\frac{4}{15}=\frac{55}{120}+\frac{32}{120}=\frac{87}{120}=\frac{29}{40}\).
2. \(\frac{11}{30}-\frac{7}{80}=\frac{88}{240}-\frac{21}{240}=\frac{67}{240}\).
3. \(\frac{19}{35}+\frac{10}{21}=\frac{57}{105}+\frac{50}{105}=\frac{107}{105}=1\frac{2}{105}\).

Ответ: а) \(1\frac{7}{60};\ \frac{14}{75};\ \frac{65}{252}\). б) \(\frac{29}{40};\ \frac{67}{240};\ 1\frac{2}{105}\).