Đáp án:
$\begin{array}{l}
1)\sqrt {9{{\left( {x - 2} \right)}^2}} - 12 = 0\\
\Rightarrow 3\left| {x - 2} \right| = 12\\
\Rightarrow \left| {x - 2} \right| = 4\\
\Rightarrow \left[ \begin{array}{l}
x - 2 = 4\\
x - 2 = - 4
\end{array} \right.\\
\Rightarrow \left[ \begin{array}{l}
x = 6\\
x = - 2
\end{array} \right.\\
\text{Vậy}\,x = 6;x = - 2\\
2)Dkxd:x \ge 3\\
\sqrt {x - 3} = 21\\
\Rightarrow x - 3 = 441\\
\Rightarrow x = 444\left( {tmdk} \right)\\
\text{Vậy}\,x = 444\\
3)Dkxd:m \ne 2\\
P = 20 - \dfrac{1}{{m - 2}}.\sqrt {{m^2} - 4m + 4} \\
= 20 - \dfrac{1}{{m - 2}}.\sqrt {{{\left( {m - 2} \right)}^2}} \\
= 20 - \dfrac{1}{{m - 2}}.\left| {m - 2} \right|\\
+ Khi:m > 2 \Rightarrow \left| {m - 2} \right| = m - 2\\
\Rightarrow P = 20 - \dfrac{1}{{m - 2}}.\left( {m - 2} \right) = 20 - 1 = 19\\
+ Khi:m < 2\\
\Rightarrow P = 20 - \dfrac{1}{{m - 2}}.\left( {2 - m} \right) = 20 + 1 = 21
\end{array}$
