Tính: A=1/4+1/28+1/70+1/130+……+1/700

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1. `A = 1/4 + 1/28 + 1/70+ 1/130 +…+ 1/700`

   `A= 1/1.4 + 1/4.7 + 1/7.10 + 1/10.13+…+ 1/25.28`

   `3A = 3/1.4 + 3/4.7 + 3/7.10 + 3/10.13 +…..+ 3/25.28`

   `3A= 1 -1/4 + 1/4 – 1/7 + 1/7- 1/10+…+ 1/25- 1/28`

   `3A= 1-1/28`

   `3A=27/28`

   `A= 27/28 :3`

   `A= 9/28`

   Vậy `A = 9/28`

    

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2. Đáp án + giải thích bước giải :

   `A = 1/4 + 1/28 + 1/70 + 1/130 + …. + 1/700`

   `-> A = 1/(1 . 4) + 1/(4 . 7) + 1/(7 . 10) + 1/(10 . 13) + …. + 1/(25 . 28)`

   `-> 3A = 3/(1 . 4) + 3/(4 . 7) + 3/(7 . 10) + 3/(10 . 13) + …. + 3/(25 . 28)`

   `-> 3A = 1 – 1/4 + 1/4 – 1/7 + 1/7 – 1/10 + 1/10-  1/13 + …. + 1/25 – 1/28`

   `-> 3A = 1 +(- 1/4 + 1/4 – 1/7 + 1/7 – 1/10 + 1/1-  1/13 + …. + 1/25) – 1/28`

   `-> 3A = 1 – 1/28`

   `-> 3A = 27/28`

   `-> A = 27/28 : 3`

   `-> A = 9/28`

   Vậy `A = 9/28`

    

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