$a) x^2 - 2x -80 = 0 $
$⇔ x^2 - 10x + 8x- 80= 0 $
$⇔ x( x + 8 ) -10 ( x + 8 )= 0$
$⇔ (x + 8) ( x - 10 ) = 0$
$⇔ x + 8 ) = 0 hay x - 10 = 0 $
$⇒ x = -8 hay x =10 $
$\text{ Vậy S = { - 8 ; 10 }}$
$b) x^2 - 12x +11 = 0 $
$⇔ x^2 - 11x - x + 11 = 0 $
$⇔ x ( x - 1) - 11 ( x - 1 ) = 0$
$⇔ ( x - 1 ) ( x -11 ) = 0 $
$ ⇔ x -1 = 0 hay x -11 = 0 $
$⇒ x = 1 hay x = 11$
$\text{ Vậy S = { 1 ; 11 }}$
$c) 4x^2 - 12x - 7 = 0 $
$⇔ 4x^2 - 14x + 2x - 7 = 0$
$⇔ 2x( 2x +1 ) -7 ( 2x + 1 ) = 0$
$⇔ ( 2x - 7 ) ( 2x + 1 ) = 0$
$⇔ 2x - 7 = 0 hay 2x + 1 = 0 $
$⇒ x = - 0,5 hay x = 3,5$
$\text{ Vậy S = { -0,5 ; 3,5 }}$
