= $\frac{21}{13}$ x $\frac{39}{17}$ x $\frac{51}{84}$
= $\frac{21}{13}$ x $\frac{39}{17}$ x $\frac{17}{28}$
= $\frac{996}{6188}$ x $\frac{14196}{6188}$ x $\frac{11271}{6188}$
= $\frac{747}{2023}$ x $\frac{11271}{6188}$
= $\frac{2241}{3332}$
chúc bn hok tốt