Giải thích các bước giải:
$(\dfrac{1}{1.2.3.4}+\dfrac{1}{2.3.4.5}+..+\dfrac{1}{7.8.9.10})x=\dfrac{119}{720}$
$\to (\dfrac{3}{1.2.3.4}+\dfrac{3}{2.3.4.5}+..+\dfrac{3}{7.8.9.10})x=\dfrac{119}{240}$
$\to (\dfrac{4-1}{1.2.3.4}+\dfrac{5-2}{2.3.4.5}+..+\dfrac{10-7}{7.8.9.10})x=\dfrac{119}{240}$
$\to (\dfrac{1}{1.2.3}-\dfrac{1}{2.3.4}+\dfrac{1}{2.3.4}-\dfrac{1}{3.4.5}+..+\dfrac{1}{7.8.9}-\dfrac{1}{8.9.10})x=\dfrac{119}{240}$
$\to (\dfrac{1}{1.2.3}-\dfrac{1}{8.9.10})x=\dfrac{119}{240}$
$\to \dfrac{119}{720}x=\dfrac{119}{240}$
$\to x=3$
