Toán Lớp 7: B=1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72

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Toán Lớp 7: B=1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72, hướng dẫn giải giúp em bài này ạ, em cảm ơn thầy cô và các bạn nhiều.

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Hiểu Vân 26 phút 2022-05-01T04:56:13+00:00 2 Answers 0 views 0

TRẢ LỜI ( 2 )

  1. $B=\dfrac{1}{2}+\dfrac{1}{6}+\dfrac{1}{12}+\dfrac{1}{20}+\dfrac{1}{30}+\dfrac{1}{42}+\dfrac{1}{56}+\dfrac{1}{72}$

    $B=\dfrac{1260}{2520}+\dfrac{420}{2520}+\dfrac{210}{2520}+\dfrac{126}{2520}+\dfrac{84}{2520}+\dfrac{60}{2520}+\dfrac{45}{2520}+\dfrac{35}{2520}$

    $B=\dfrac{1260+420+210+126+84+60+45+35}{2520}$

    $B=\dfrac{2240}{2520}$

    $B=\dfrac{8}{9}$

     

  2.  B = 1/2 +1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56 + 1/72

    = 1/( 1. 2) + 1/( 2 . 3) + 1/( 3 . 4) + 1/( 4 . 5) + 1/( 5 . 6) + 1/( 6 . 7) + 1/( 7 . 8) + 1/( 8 . 9)

    = ( 1 – 1/2) + ( 1/2 – 1/3) + ( 1/3 – 1/4) + ( 1/4 – 1/5) + ( 1/5 – 1/6) + ( 1/6 – 1/7) + ( 1/7 – 1/8) + ( 1/8 – 1/9)

    = 1 – 1/2 + 1/2 – 1/3 + 1/3 – 1/4 + 1/4 – 1/5 + 1/5 – 1/6 + 1/6 – 1/7 + 1/7 – 1/8 + 1/8 – 1/9

    = 1 – ( 1/2 – 1/2) – ( 1/3 – 1/3) – ( 1/4 – 1/4) – ( 1/5 – 1/5) – ( 1/6 – 1/6) – ( 1/7 – 1/7) – ( 1/8 – 1/8) – 1/9

    = 1 – 0 – 0 – 0 – 0 – 0 – 0 – 0 – 1/9

    = 1 – 1/9

    = 9/9 – 1/9

    = 8/9

    Vậy B = 8/9

    #caty09

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