`K = x - (9x^2)/25`

`K = -(9/25x^2 -x)`

`K = -9/25(x^2 - 25/9x)`

`K = -9/25(x^2 - 2 . 25/18x + 625/324 - 625/324)`

`K = -9/25(x - 25/18)^2 +25/36`

Ta có : `(x - 25/18)^2 ≥0∀x`

`=> -9/25(x - 25/18)^2 ≤0∀x`

`=> -9/25(x - 25/18)^2 +25/36 ≤ 25/36∀x`

Dấu "=" xảy ra `⇔x-25/18=0`

                        `⇔x=25/18`

Vậy `Kmax = 25/36⇔x=25/18`