B= 1/1.5+1/5.9+…+1/2018.2020 C= 1/10.12+1/12.14+…+1/998.1000 giúp mk với mọi nười ơi

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1. Đáp án: đề câu B sai á, sửa lại nha

    $B = \dfrac{1}{1.5}+\dfrac{1}{5.9} + …+ \dfrac{1}{2018.2022}$

   $ =\dfrac{1}{4}(1 – \dfrac{1}{5} +\dfrac{1}{5} -\dfrac{1}{9} +….+\dfrac{1}{2018} – \dfrac{1}{2022}_$

   $ = \dfrac{1}{4} . (1 – \dfrac{1}{2022})$

   $= \dfrac{1}{4} . (\dfrac{2022}{2022}- \dfrac{1}{2022}$

   $=\dfrac{1}{4} . \dfrac{2021}{2022}$

   $ =\dfrac{2021}{8088}$

   $C = \dfrac{1}{10 . 12} + \dfrac{1}{12.14} +…+\dfrac{1}{998 .1000}$

   $ = \dfrac{1}{2} . (\dfrac{1}{10}-\dfrac{1}{12}+\dfrac{1}{12} -\dfrac{1}{14} +…+\dfrac{1}{998} – \dfrac{1}{1000}$

   $ =\dfrac{1}{2} . (\dfrac{1}{10} – \dfrac{1}{1000})$

   $ = \dfrac{1}{2} .(\dfrac{100}{1000} -\dfrac{1}{1000})$

   $ =\dfrac{1}{2} . \dfrac{99}{1000}$

   $ = \dfrac{99}{2000}$

    

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2. Giải thích các bước giải:

   Sửa đề:

   `B = 1/1.5 + 1/5.9 +…+ 1/2018.2022`

   `= 1/4 . (4/1.5 + 4/5.9 +…+ 4/2018.2022)`

   `= 1/4 . (1 – 1/5 + 1/5 – 1/9 +…+ 1/2018 – 1/2022)`

   `= 1/4 . (1 – 1/2022)`

   `= 1/4 . 2021/2022 = 2021/8088`

   `C= 1/10.12 + 1/12.14 +…+ 1/998.1000`

   `= 1/2 . (2/10.12 + 2/12.14 +…+ 2/998.1000)`

   `= 1/2 . (1/10 – 1/12 + 1/12 – 1/14 +…+ 1/998 – 1/1000)`

   `= 1/2 . (1/10 – 1/1000)`

   `= 1/2 . 99/1000 = 99/2000`

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