Hướng dẫn trả lời:
$\text{25 - $x^2$ = (x - 5).(2x + 7).}$
$\text{→ 25 - $x^2$ = $2x^2$ - 3x - 35.}$
$\text{→ 25 - $x^2$ - ($2x^2$ - 3x - 35) = 0.}$
$\text{→ 25 - $x^2$ - $2x^2$ + 3x + 35 = 0.}$
$\text{→ $-3x^2$ + 3x + 60 = 0.}$
$\text{→ -3.($x^2$ - x - 20) = 0.}$
$\text{→ $x^2$ - x - 20 = 0.}$
$\text{→ $x^2$ - 5x + 4x - 20 = 0.}$
$\text{→ ($x^2$ - 5x) + (4x - 20) = 0.}$
$\text{→ x.(x - 5) + 4.(x - 5) = 0.}$
$\text{→ (x - 5).(x + 4) = 0.}$
$\text{→ \(\left[ \begin{array}{l}x - 5= 0.\\x + 4 = 0.\end{array} \right.\)}$
$\text{→ \(\left[ \begin{array}{l}x = 5.\\x = - 4.\end{array} \right.\)}$
$\text{Vậy tập nghiệm phương trình S = {5; -4}.}$
