(x-2)^3 + (5-2x)^3 = 0

`[` Bạn tham khảo `]`

`(x-2)^3 + (5-2x)^3 = 0`

`<=> x^3 - 6x^2 + 12x- 8 + 125 - 150x + 60x^2 - 8x^3 = 0`

`<=> (x^3 - 8x^3) - (6x^2 - 60x^2) + (12x - 150x) - (8 - 125) = 0`

`<=> -7x^3 + 54x^2 - 138x + 117 = 0`

`<=> -(7x^3 - 54x^2 + 138x - 117) = 0`

`<=> 7x^3 - 54x^2 + 138x - 117 = 0`

`<=> 7x^3 - 21x^2 - 33x^2 + 99x + 39x - 117 = 0`

`<=> 7x^2.(x - 3) - 33x.(x - 3) + 39.(x - 3) = 0`

`<=> (x -3).(7x^2 - 33x + 39) = 0`

mà: `7x^2 - 33x + 39 = 7.(x^2- 33/7x + 39/7) =7.(x^2- 2.x . 33/14 + 1089/196 + 3/196 ) = 7.(x - 33/14)^2 + 3/28 > 0`

`=> x - 3 =0`

`<=> x = 3`

Vậy `S = {3}`

`#20`