` ( 5x -2 )^2 =4 ( 2-x)^2`

`<=> (5x )^2 -2.5x.2 + 2^2 =4 ( 2^2 -2.x.2 + x^2 )`

`<=> 25x^2 -20x + 4 = 4 ( 4 -4x + x^2 )`

`<=> 25x^2 -20x + 4 = 16 - 16x -4x^2`

`<=> ( 25x^2 -4x^2 ) - 20x + 16x + 4 -16=0`

`<=> 21x^2 -4x -12=0`

`<=> 21x^2 -18x +14x -12=0`

`<=>(21x^2 -18x )+ (14x-12 )=0`

`<=>3x (7x -6 ) + 2 (7x -6 )+0`

`<=> ( 7x -6 )(3x +2 )=0`

`<=> [ ( 7x-6=0), ( 3x + 2 =0):}`

`<=> [ ( 7x = 6 ),( 3x =-2 ):}`

`<=>[ ( x=6/7 ), ( x= -2/3 ):}`

Vậy `S= { -2/3 ; 6/7 }`