Решите:а)5/21+9/14 б)5/21:9/14 в)5/21-9/14*5/18

А) $\frac{5}{21} + \frac{9}{14} = \frac{5*2}{21*2} + \frac{9*3}{14*3} = \frac{10}{42} + \frac{27}{42} = \frac{37}{42}$
б) $\frac{5}{21} : \frac{9}{14} = \frac{5}{21} * \frac{14}{9} = \frac{70}{189} = \frac{10}{27}$
в) 1) $\frac{9}{14} * \frac{5}{18} = \frac{45}{252} = \frac{5}{28}$
2) $\frac{5}{21} - \frac{5}{28} = \frac{5*4}{21*4} - \frac{5*3}{28*3} = \frac{20}{84} - \frac{15}{84} = \frac{5}{84}$