6. Найди значение выражения a/21+14/a при а = 21; а = 14; a = 7.

1. При a = 21:

   $$\frac{21}{21} + \frac{14}{21} = 1 + \frac{2}{3} = \frac{3}{3} + \frac{2}{3} = \frac{5}{3} = 1\frac{2}{3}$$

2. При a = 14:

   $$\frac{14}{21} + \frac{14}{14} = \frac{2}{3} + 1 = \frac{2}{3} + \frac{3}{3} = \frac{5}{3} = 1\frac{2}{3}$$

3. При a = 7:

   $$\frac{7}{21} + \frac{14}{7} = \frac{1}{3} + 2 = \frac{1}{3} + \frac{6}{3} = \frac{7}{3} = 2\frac{1}{3}$$

Ответ: При а = 21: 1 2/3; при а = 14: 1 2/3; при а = 7: 2 1/3.