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

`(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`