Đáp án:
Giải thích các bước giải:
$\dfrac{12}{84}+\dfrac{12}{210}+...+\dfrac{12}{2100}$
$ $
$=\dfrac{4}{28}+\dfrac{4}{70}+...+\dfrac{4}{700}$
$ $
$=\dfrac{4}{4.7}+\dfrac{4}{7.10}+...+\dfrac{4}{25.28}$
$ $
$=\dfrac{4}{3}.(\dfrac{3}{4.7}+\dfrac{3}{7.10}+...+\dfrac{3}{25.28})$
$ $
$=\dfrac{4}{3}.(\dfrac{1}{4}-\dfrac{1}{28})$
$ $
$=\dfrac{4}{3}.\dfrac{3}{14}$
$ $
$=\dfrac{2}{7}$
