\(\dfrac{7}{2}\) < a < \(\dfrac{14}{3}\)
\(\dfrac{7\times3}{2\times3}\) < a < \(\dfrac{14\times2}{3\times2}\)
\(\dfrac{21}{6}\) < \(\dfrac{6\times a}{6}\) < \(\dfrac{28}{6}\)
21 < 6x a < 28
vì 21 < 22 < 23 < 24 < 25 < 26 < 27 < 28
6 x a = 22; 23; 24; 25; 26; 27
a = 11/3; 23/6; 4; 25/6; 13/3; 27/6
vì a là số tự nhiên nên a = 4