Tính S=[tex]\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{2017.2018.2019}[/tex]
$ S = \frac{1}{1 . 2 . 3} + \frac{1}{2 . 3 . 4} + ... + \frac{1}{2017 . 2018 . 2019} \\ = \frac{1}{2} . \left ( \frac{2}{1 . 2 . 3} + \frac{2}{2 . 3 . 4} + ... + \frac{2}{2017 . 2018 . 2019} \right ) \\ = \frac{1}{2}\left (\frac{1}{1 . 2} - \frac{1}{2 . 3} + \frac{1}{2 . 3} - \frac{1}{3 . 4} + ... + \frac{1}{2017 . 2018} - \frac{1}{2018 . 2019} \right )\\ = \frac{1}{2}\left ( \frac{1}{2} - \frac{1}{2018 . 2019} \right ) \\ = ... $
[TẶNG BẠN] TRỌN BỘ Bí kíp học tốt 08 môn
