11. $\sqrt{16 - 8x + x^2} - 5 = 7 (Đk: x ∈ R)$
$⇔ \sqrt{(x - 4)^2} - 5 = 7$
$⇔ |x - 4| = 12$
⇔ \(\left[ \begin{array}{l}x-4 = 12\\x - 4 = -12\end{array} \right.\)
⇔ \(\left[ \begin{array}{l}x = 16\\x = -8\end{array} \right.\)
12. $\sqrt{25(x-3)^2} - 15 = 0 (ĐK: x ∈ R)$
⇔$\sqrt{[5(x-3)]^2} = 15$
⇔ $5|x - 3| = 15$
⇔ $x - 3 = 3$
⇔ \(\left[ \begin{array}{l}x-3 = 3\\x - 3 = -3\end{array} \right.\)
⇔ \(\left[ \begin{array}{l}x = 6\\x = 0\end{array} \right.\)
13. $\sqrt{9x^2} = |-12| (ĐK: x ∈ R)$
$⇔ \sqrt{(x)^2} = 12$
$⇔ 3|x| = 12$
$⇔ |x| = 4$
⇔ \(\left[ \begin{array}{l}x = 4\\x = - 4\end{array} \right.\)
14. $\sqrt{x^2} = 7 (ĐK: x ∈ R)$
⇔ $|x| = 7$
⇔\(\left[ \begin{array}{l}x = 7\\x = - 7\end{array} \right.\)
