Tính `A=4/21+4/77+4/165+4/285+4/437+4/621`.

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1. `A = 4/21 + 4/77 + 4/165 + 4/285 + 4/437 + 4/621`

   `= 4/(3.7) + 4/(7.11) + 4/(11.15) + 4/(15.19) + 4/(19.23) + 4/(23.27)`

   `= 1/3 – 1/7 + 1/7 – 1/11 + 1/11 – 1/15 + 1/15 – 1/19 + 1/19 – 1/23 + 1/23 – 1/27`

   `= 1/3 – 1/27`

   `= 9/27-1/27`

   `= 8/27`

   Vậy `A = 8/27`

    

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2. Đáp án:

    

   Giải thích các bước giải:

   A=`4/21`+`4/77`+`4/165`+`4/285`+`4/437`+`4/621`

   A=`4/3.7`+`4/7.11`+`4/11.15`+`4/15.19`+`4/19.23`+`4/23.27`

   A=`1/3`-`1/7`+`1/7`-`1/11`+`1/11`-`1/15`+`1/15`-`1/19`+`1/19`-`1/23`+`1/23`-`1/27`

   A=`1/3`-`1/27`

   A=`8/27`

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