Tính 1. A= 1/2+1/6+1/12+/1/20+1/30+1/42+1/56 2. B = 3/2+5/6+7/12+-9/20+11/30-13/42+15/56

Tính
1. A= 1/2+1/6+1/12+/1/20+1/30+1/42+1/56
2. B = 3/2+5/6+7/12+-9/20+11/30-13/42+15/56

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  1. Đáp án:

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

    =1-1/8

    =7/8

    2.B= 

    = (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+

    =

     

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  2. Đáp án:

    `A= 1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56 `

    `A = 1/(1.2) + 1/(2 . 3) + 1/(3.4) + 1/(4.5) + 1/(5.6) + 1/(6.7) + 1/(7.8)`

    `A = 1/1 – 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`

    `A = 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`

    `A = 1 + 0 + 0 + 0 + 0 + 0 + 0 – 1/8`

    `A = 1 – 1/8`

    `A = 8/8 – 1/8 `

    `A = 7/8`

    `B = 3/2+5/6+7/12+-9/20+11/30-13/42+15/56`

    Sửa lại đề:

    `B = 3/2 – 5/6 + 7/12 – 9/20 + 11/30 – 13/42 + 15/56`

    `B = 3/(1.2) –  5/(2.3) + 7/(3.4) – 9/(4 . 5) + 11/(5.6) – 13/(6.7) + 15/(7.8)`

    `B = 3 – 3/2 -5/2 -(- 5/3) + 7/3 – 7/4 – 9/4 -(- 9/5) + 11/5 – 11/6 – 13/6 -(- 13/7) + 15/7 – 15/8`

    `B = 3 – 3/2 -5/2 + 5/3 + 7/3 – 7/4 – 9/4 + 9/5 + 11/5 – 11/6 – 13/6 + 13/7 + 15/7 – 15/8`

    `B = 3 + (-3/2 – 5/2) + (5/3 + 7/3) + (-7/4 – 9/4) + (9/5 + 11/5) + (-11/6 – 13/6) +( 13/7  + 15/7) – 15/8`

    `B = 3 + (-4) + 4 + (-4) + 4 + (-4) + 4 – 15/8`

    `B = 3 + [(-4) + 4] +[ (-4) + 4] +[ (-4) + 4] – 15/8`

    `B = 3 + 0 + 0 + 0 – 15/8`

    `B = 3 – 15/8`

    `B = 24/8 – 15/8`

    `B = 9/8`

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