Tìm x `x^2-9-5x(x-3)=0`

`x^2 - 9 - 5x(x - 3) = 0`

`<=> x^2 - 9 - (5x^2 - 15x) = 0`

`<=> x^2 - 9 - 5x^2 + 15x = 0`

`<=> -4x^2 + 15x - 9 = 0`

`<=> -(4x^2 - 15x - 9) = 0`

`<=> -(4x^2 - 3x - 12x - 9) = 0`

`<=> -[x(4x - 3) - 3(4x - 3)] = 0`

`<=> -(x - 3)(4x - 3) = 0`

`<=> -(x - 3) = 0` `hoặc` `4x - 3 = 0`

`<=> 3 - x = 0` `hoặc` `4x = 3`

`<=> x = 3` `hoặc` `x = 3/4`

`Vậy: x \in {3; 3/4}`