4 9 21 14

8) $$\frac{4}{21} + \frac{9}{14} = \frac{4 \cdot 2}{21 \cdot 2} + \frac{9 \cdot 3}{14 \cdot 3} = \frac{8}{42} + \frac{27}{42} = \frac{8+27}{42} = \frac{35}{42} = \frac{5}{6}$$

Ответ: $$\frac{5}{6}$$