Đáp án + Giải thích các bước giải:
H = $\dfrac{10}{56}$ + $\dfrac{10}{140}$ + $\dfrac{10}{260}$ + ....... + $\dfrac{10}{1400}$
H = $\dfrac{5}{28}$ + $\dfrac{5}{70}$ + $\dfrac{5}{130}$ + $\dfrac{5}{700}$
$\dfrac{3H}{5}$ = $\dfrac{3}{4}$ x ${7}$ + $\dfrac{3}{4}$ x ${10}$ + $\dfrac{30}{10}$ x ${13}$ ....... + $\dfrac{30}{25}$ x ${28}$
$\dfrac{3H}{5}$ x =$\dfrac{1}{4}$ + $\dfrac{1}{7}$ - $\dfrac{1}{10}$ - $\dfrac{1}{13}$ + ........ + $\dfrac{1}{25}$- $\dfrac{1}{28}$
$\dfrac{3H}{5}$ = $\dfrac{1}{4}$ - $\dfrac{1}{28}$
$\dfrac{3H}{5}$ = $\dfrac{3}{14}$
${H}$ = $\dfrac{3}{14}$ x $\dfrac{5}{3}$
${H}$ = $\dfrac{5}{14}$
