Đáp án:
Giải thích các bước giải:
$A=\dfrac{1}{6}+\dfrac{1}{12}+\dfrac{1}{20}+...+\dfrac{1}{380}$
$ $
$A=\dfrac{1}{2.3}+\dfrac{1}{3.4}+\dfrac{1}{4.5}...+\dfrac{1}{19.20}$
$ $
$A=\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+...+\dfrac{1}{19}-\dfrac{1}{20}$
$ $
$A=\dfrac{1}{2}-\dfrac{1}{20}$
$ $
$A=\dfrac{10}{20}-\dfrac{1}{20}$
$ $
$A=\dfrac{9}{20}$
