`\text{a)}`
`2x + 4/5x = 7/25`
`-> ( 2+4/5)x = 7/25`
`-> 14/5x = 7/25`
`-> x = 7/25 : 14/5`
`-> x = 7/25 . 5/14 =1/10`
Vậy `x = 1/10`
$\\$
`\text{b)}`
`3 - |x+3/7| = 56/21`
`-> |x+3/7| = 3 - 56/21`
`-> |x+3/7| = 1/3`
`->` \(\left[ \begin{array}{l}x+\dfrac{3}{7}= \dfrac{1}{3}\\x+\dfrac{3}{7} =\dfrac{-1}{3}\end{array} \right.\)
`->` \(\left[ \begin{array}{l}x=\dfrac{-2}{21}\\x=\dfrac{-16}{21}\end{array} \right.\)
Vậy `x \in {-2/21 ; -16/21}`
$\\$
`\text{c)}`
`(2x - 2,4)^2 : 3/4 - 1/3 =1`
`-> (2x - 2,4)^2 : 3/4 = 1 +1/3 = 4/3`
`-> (2x - 2,4)^2 = 4/3 . 3/4 =1`
`->` \(\left[ \begin{array}{l}2x -2,4 =1\\2x-2,4=-1\end{array} \right.\)
`->` \(\left[ \begin{array}{l}x=\dfrac{17}{10}\\x=\dfrac{7}{10}\end{array} \right.\)
Vậy `x \in { 17/10 ; 7/10}`
