Đáp án:
Giải thích các bước giải:
$F=\dfrac{1}{18}+\dfrac{1}{54}+...+\dfrac{1}{990}$
$ $
$F=\dfrac{1}{3.6}+\dfrac{1}{6.9}+...+\dfrac{1}{30.33}$
$ $
$⇒3F=\dfrac{3}{3.6}+\dfrac{3}{6.9}+...+\dfrac{3}{30.33}$
$ $
$⇒3F=\dfrac{1}{3}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{9}+...+\dfrac{1}{30}-\dfrac{1}{33}$
$ $
$⇒3F=\dfrac{1}{3}-\dfrac{1}{33}$
$ $
$⇒3F=\dfrac{10}{33}$
$ $
$⇒F=\dfrac{10}{33}:3=\dfrac{10}{99}$
