A=2/2.5+2/5.8+2/8.11+2/11.14+2/14.17+2/17.20

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1. Đáp án:

   `A = 3/10`

   Giải thích các bước giải:

   `A = 2/2.5 + 2/5.8 + 2/8.11 + 2/11.14 + 2/14.17 + 2/17.20`

   `= 2/3 . ( 3/2.5 + 3/5.8 + 3/8.11 + 3/11.14 + 3/14.17 + 3/17.20 )`

   `= 2/3 . ( 1/2-1/5+1/5-1/8+1/8-1/11+1/11-1/14+1/14-1/17+1/17-1/20 )`

   `= 2/3 . ( 1/2-1/20 ) = 2/3 . 9/20 = 3/10`

   Vậy `A = 3/10`

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2. A=2/2.5+2/5.8+2/8.11+2/11.14+2/14.17+2/17.20

   A=3/2(3/2.5+3/5.8+3/8.11+3/11.14+3/14.17+3/17.20

   A=3/2(1/2-1/5+1/5-1/8+…..+1/17-1/20)

   A=3/2(1/2-1/20)

   A=3/2(10/20-1/20)

   A=3/2.9/20

   A=27/40

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