Đáp án:
S=$\frac{265}{357}$
Giải thích các bước giải:
S=(1-$\frac{1}{21}$)(1-$\frac{1}{28}$)...(1- $\frac{1}{1326}$)
=$\frac{20}{21}$. $\frac{27}{28}$.$\frac{35}{36}$... $\frac{1325}{1326}$
= $\frac{40}{42}$. $\frac{54}{56}$...$\frac{2650}{2652}$
=$\frac{5.8}{6.7}$. $\frac{6.9}{7.8}$. $\frac{7.10}{8.9}$... $\frac{50.53}{51.52}$
=$\frac{5.53}{7.51}$= $\frac{265}{357}$
