a) `x^2-x = 20`
`<=> x^2 - 5x + 4x - 20 =0`
`<=> x(x-5) + 4(x-5) =0`
`<=> (x+4)(x-5)=0`
`<=>` \(\left[ \begin{array}{l}x=-4\\x=5\end{array} \right.\)
b) `4x^2+19x+9x=0
`<=> 4x^2+28x =0`
`<=> x^2+7x=0`
`<=> x(x+7) =0`
`<=>` \(\left[ \begin{array}{l}x=0\\x=-7\end{array} \right.\)
c) `(4x-3)(4x+3) - 5x(3x-1) = -7`
`<=> 16x^2 - 9 - 15x^2 + 5x + 7=0`
`<=> x^2 + 5x - 2 =0`
Có: `\Delta = 5^2 - 4.1.(-2) = 33 > 0`
`=>` PT có 2 nghiệm phân biệt: `x_(1,2) = (-5\pm\sqrt33)/2`
