Bạn tham khảo :
$\dfrac{1}{6}$ +$\dfrac{1}{12}$ + $\dfrac{1}{20}$ + $\dfrac{1}{30}$ + $\dfrac{1}{42}$ + $\dfrac{1}{56}$
=$\dfrac{1}{1.3}$ + $\dfrac{1}{3.4}$ + $\dfrac{1}{4.5}$ + $\dfrac{1}{5.6}$ + $\dfrac{1}{6.7}$+$\dfrac{1}{7.8}$
= $\dfrac{1}{2}$ - $\dfrac{1}{3}$ + $\dfrac{1}{3}$ - $\dfrac{1}{4}$ + $\dfrac{1}{4}$ - $\dfrac{1}{5}$ + $\dfrac{1}{5}$- $\dfrac{1}{6}$ + $\dfrac{1}{6}$ - $\dfrac{1}{7}$ + $\dfrac{1}{7}$ - $\dfrac{1}{8}$
= $\dfrac{1}{2}$ - $\dfrac{1}{8}$
= $\dfrac{4}{8}$ - $\dfrac{1}{8}$
= $\dfrac{3}{8}$
