$\text{a.$4^{2}$ - 25 - (2x - 5)(2x + 7) = 0 }$
$\text{ (2x - 5)(2x + 5) - (2x - 5)(2x + 7) = 0 }$
$\text{ (2x - 5)[(2x + 5) - (2x + 7)] = 0 }$
$\text{ (2x - 5)(2x + 5 - 2x - 7) = 0 }$
$\text{ (2x - 5)(2x + 5 - 2x - 7) = 0 }$
$\text{ (-2)(2x - 5) = 0 }$
$\text{⇒ (-2) = 0 hoặc 2x - 5 = 0 }$
$\text{ Vô lý x = $\dfrac{5}{2}$ }$
$\text{b) $x^{3}$ + 27 + (x + 3)(x - 9) = 0 }$
$\text{ (x + 3)($x^{2}$ - 3x +9) + (x + 3)(x - 9) = 0 }$
$\text{ (x + 3)($x^{2}$ - 3x +9 + x - 9) = 0 }$
$\text{ (x + 3)($x^{2}$ - 2x) = 0 }$
$\text{ x(x + 3)(x - 2) = 0 }$
$\text{x = 0 hoặc x + 3 = 0 hoặc x - 2 = 0 }$
$\text{x = 0 hoặc x = -3 hoặc x = 2 }$
c) $\text{$2x^{3}$ + $3x^{2}$ + 2x - 3 = 0}$
$\text{( $2x^{3}$ + 2x) + ($3x^{2}$- 3) = 0}$
$\text{2x($x^{2}$ + 1) + 3($x^{2}$ + 1) = 0}$
$\text{($x^{2}$ + 1)(2x +3) = 0}$
⇒ $\text{$x^{2}$ + 1= 0 hoặc 2x +3 = 0}$
⇒ $\text{$x^{2}$ = -1 ( Vô lý) hoặc x = $\dfrac{-3}{2}$ }$
