$\frac{2}{3}$ + $\frac{2}{15}$ + $\frac{2}{35}$ + $\frac{2}{63}$
= $\frac{2}{1.3}$ + $\frac{2}{3.5}$ + $\frac{2}{5.7}$ + $\frac{2}{7.9}$
= 1 - $\frac{1}{3}$ + $\frac{1}{3}$ - $\frac{1}{5}$ + $\frac{1}{5}$ - $\frac{1}{7}$ + $\frac{1}{7}$ - $\frac{1}{9}$
= 1 - $\frac{1}{9}$
= $\frac{8}{9}$
Chúc học tốt!!!