1,tính hợp lí a,8/7×3/4×7/8×12/15×2 b,(98/99+99/100+100/101)x(1/12-1/3+1/4) 2,tính nhanh S=1/6+1/12+1/20+1/30+1/42+1/56 3,tính S=1/1x2x3+1/2x3x4+1/

Question

1,tính hợp lí
a,8/7×3/4×7/8×12/15×2
b,(98/99+99/100+100/101)x(1/12-1/3+1/4)
2,tính nhanh
S=1/6+1/12+1/20+1/30+1/42+1/56
3,tính
S=1/1x2x3+1/2x3x4+1/3x4x5+…………+1/98x99x100
làm nhanh mình đang cần gấp nhé

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Aubrey 1 giờ 2021-09-07T12:34:07+00:00 2 Answers 0 views 0

Answers ( )

    0
    2021-09-07T12:35:18+00:00

    Đáp án:

     

    Giải thích các bước giải:

    a,`8/7.(3/4).(7/8).(12/15).2`

    `=(8/7).(7/8).(3/4.(12/15).2`

    `=1.2.(3/4).(4/5)`

    `=2.(3/5)=6/5`

    b,`(98/99+99/100+100/101).(1/12-1/3+1/4)`

    `=(98/99+99/100+100/101).(1/12-1/12)`

    `=(98/99+99/100+100/101).0=0`

    2,

    `S=1/6+1/12+1/20+1/30+1/42+1/56`

    `=1/(2.3)+1/(3,4)+1/(4.5)+1/(5.6)+1/(6.7)+1/(7.8)`

    `=1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8`

    `=1/2-1/8=3/8`

    `S=1/(1.2.3)+1/(2.3.4)+1/(3.4.5)+…………+1/(98.99.100)`

    `2S=2/(1.2.3)+2/(2.3.4)+2/(3.4.5)+…………+2/(98.99.100)`

    `2S=1/(1.2)-1/(2.3)+1/(2.3)-1/(3.4)+1/(3.4)-1/(4.5)+…….+1/(98.99)-1/(99.100)`

    `2S=1/2-1/9900=4949/9900`

    `=>S=4949/19800`

    dấu . là dấu nhân nhé

    0
    2021-09-07T12:35:22+00:00

    Đáp án:

     

    Giải thích các bước giải:

     Bài 1:

    a, 8/7 x 3/4 x 7/8 x 12/15 x 2

    = (8/7 x 7/8) x (3/4 x 12/15) x 2

    = 1 x 1 x 2

    = 2

    b, (98/99+99/100+100/101) x (1/12-1/3+1/4)

    = (98/99+99/100+100/101) x (1/12 – 4/12 + 3/12)

    = (98/99+99/100+100/101) x 0

    = 0

    Bài 2:

    S= 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56

      = 1/2.3 + 1/3.4 + 1/4.5 + 1/5.6 + 1/6.7 + 1/7.8

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

      = 1/2 – 1/8

      = 4/8 – 1/8

      = 3/8

    Vậy S = 3/8

    Bài 3:

       S=1/1x2x3+1/2x3x4+1/3x4x5+…………+1/98x99x100

    4S = 1/1x2x3x4 + 1/2x3x4x(5-1) + ……+1/98x99x100x(101-97)

    4S =  1/1x2x3x4  + 1/2x3x4x5 – 1/1x2x3x4+ …. 1/98x99x100x101- 1/97x98x99x1004

      S =  1/98x99x100x101

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