Mn hộ e m vs ak đúng có ctlhn

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Mn hộ e m vs ak đúng có ctlhn
mn-ho-e-m-vs-ak-dung-co-ctlhn

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Cherry 4 years 2020-11-01T04:54:50+00:00 1 Answers 63 views 0

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  1. Nem
    0
    2020-11-01T04:55:59+00:00
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    Đáp án: a.$A=1-\dfrac1{a+1}$

                 b.$B=\dfrac{-46807}{16380}$

    Giải thích các bước giải:

    a.Ta có:

    $A=\dfrac{1}{1.2}+\dfrac{1}{2.3}+\dfrac{1}{3.4}+…+\dfrac{1}{a(a+1)}$

    $\to A=\dfrac{2-1}{1.2}+\dfrac{3-2}{2.3}+\dfrac{4-3}{3.4}+…+\dfrac{a+1-a}{a(a+1)}$

    $\to A=\dfrac11-\dfrac12+\dfrac12-\dfrac13+\dfrac13-\dfrac14+…+\dfrac1a-\dfrac1{a+1}$

    $\to A=\dfrac11-\dfrac1{a+1}$

    $\to A=1-\dfrac1{a+1}$

    b.Ta có:

    $B=\dfrac15-\dfrac34+\dfrac57-\dfrac2{11}+\dfrac7{13}-\dfrac9{10}-\dfrac73+\dfrac2{11}-\dfrac59+\dfrac37-\dfrac15$

    $\to B=(\dfrac15-\dfrac15)-\dfrac34+(\dfrac57+\dfrac37)+(\dfrac2{11}-\dfrac2{11})+\dfrac7{13}-\dfrac9{10}-\dfrac73-\dfrac59$

    $\to B=0-\dfrac34+\dfrac{8}{7}+0+\dfrac7{13}-\dfrac9{10}-\dfrac73-\dfrac59$

    $\to B=-\dfrac34+\dfrac{8}{7}+\dfrac7{13}-\dfrac9{10}-\dfrac73-\dfrac59$

    $\to B=-\dfrac{12285}{16380}+\dfrac{18720}{16380}+\dfrac{8820}{16380}-\dfrac{14742}{16380}-\dfrac{38220}{16380}-\dfrac{9100}{16380}$

    $\to B=\dfrac{-46807}{16380}$

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