1.Tìm x biết a)x+2/5=-1/3 b)5/7-x=-9/21 c)-x-1/9=-2/45 d)x+5/6=11/24 2.Tính A=5^2/1.6+5^2/6.11+…+5^2/26.31

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1. Đáp án:

   1.

   a) `x=-11/15`

   b) `x= 8/7`

   c) `x = -1/15`

   d) `x= -3/8`

   2. `A= 150/31`

   Giải thích các bước giải:

   1.

   a) `x + 2/5 = -1/3`

   `to x= -1/3 – 2/5`

   `to x= -11/15`

   Vậy `x = -11/15`

   b) `5/7 – x = -9/21`

   `to x= 5/7 – (-9/21)`

   `to x= 5/7 + 9/21`

   `to x= 8/7`

   Vậy `x= 8/7`

   c) `-x – 1/9 = -2/45`

   `to -x = -2/45 + 1/9`

   `to -x= 1/15`

   `to  x= -1/15`

   Vậy `x = -1/15`

   d) `x + 5/6 = 11/24`

   `to x= 11/24 – 5/6`

   `to x= -3/8`

   Vậy `x= -3/8`

   2.

   `A= 5^2/1.6 + 5^2/6.11 +…+ 5^2/26.31`

   `= 5 ( 5/1.6 + 5/6.11 + …+5/26.31)`

   `= 5 ( 1 – 1/6 + 1/6- 1/11 +…+1/26 – 1/31)`

   `= 5. ( 1-1/31) = 5. 30/31`

   `= 150/31`

   Vậy `A= 150/31`

    

    

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2. 1.

   a) `x + 2/5 = -1/3`

   `=> x= -1/3 – 2/5`

   `=> x= -11/15`

   Vậy `x = -11/15`

   b) `5/7 – x = -9/21`

   `=> x= 5/7 – (-9/21)`

   `=> x= 5/7 + 9/21`

   `=> x= 8/7`

   Vậy `x= 8/7`

   c) `-x – 1/9 = -2/45`

   `=> -x = -2/45 + 1/9`

   `=> -x= 1/15`

   `=> x= -1/15`

   Vậy `x = -1/15`

   d) `x + 5/6 = 11/24`

   `=> x= 11/24 – 5/6`

   `=> x= -3/8`

   Vậy `x= -3/8`

   2.

   `A= 5^2/1.6 + 5^2/6.11 +…+ 5^2/26.31`

   `A= 5( 5/1.6 + 5/6.11 + …+5/26.31)`

   `A= 5 ( 1 – 1/6 + 1/6- 1/11 +…+1/26 – 1/31)`

   `A= 5. ( 1-1/31)`

   `A= 5. 30/31`

   `A= 150/31`

   Vậy `A= 150/31`

    

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