Bài giải :

A =$\left( 1-\frac{1}{21} \right)\left( 1-\frac{1}{28} \right)\left( 1-\frac{1}{36} \right)...\left( 1-\frac{1}{1326} \right)$

A = $\left( 1-\frac{2}{42} \right)\left( 1-\frac{2}{56} \right)\left( 1-\frac{2}{72} \right)...\left( 1-\frac{2}{2652} \right)$

A = $\left( 1-\frac{2}{6.7} \right)\left( 1-\frac{2}{7.8} \right)\left( 1-\frac{2}{8.9} \right)...\left( 1-\frac{2}{51.52} \right)$

A = $\frac{5.8}{6.7}.\frac{6.9}{7.8}.\frac{7.10}{8.9}...\frac{50.53}{51.52}$

A = $\frac{5.6.7...50}{6.7.8...51}$ . $\frac{8.9.10...53}{7.8.9...52}$

A = $\frac{5}{51}.\frac{53}{7}$ 

A = $\frac{265}{357}$