Bài 4 : Tìm x , biết $\frac{3x}{2.5}$+ $\frac{3x}{5.8}$ +$\frac{3x}{8.11}$+$\frac{3x}{11.14}$=$\frac{1}{21}$

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1. `(3x)/(2.5) + (3x)/(5.8) +(3x)/(8.11) + (3x)/(11.14) = 1/21`

   `=> x . (3/(2.5) + 3/(5.8) + 3/(8.11) + 3/(11.14)) = 1/21`

   `=> x . (1/2 – 1/5 + 1/5 – 1/8 + 1/8 – 1/11 + 1/11 – 1/14) = 1/21`

   `=> x . (1/2 – 1/14) = 1/21`

   `=> x . 3/7 = 1/21`

   `=> x = 1/21 : 3/7`

   `=> x = 1/9`

   Vậy `x = 1/9`

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2. Đáp án + Giải thích các bước giải:

   `(3x)/2.5+(3x)/5.8+(3x)/8.11+(3x)/11.14=1/21`

   `=>x(3/2.5+3/5.8+3/8.11+3/11.14)=1/21`

   `=>x(1/2-1/5+1/5-1/8+1/8-1/11+1/11-1/14)=1/21`

   `=>x(1/2-1/14)=1/21`

   `=>x(7/14-1/14)=1/21`

   `=>3/7x=1/21`

   `=>x=1/21:3/7`

   `=>x=1/21. 7/3`

   `=>x=1/9`

     

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