Đáp án:
$\begin{array}{l}
a)\left| {x - 2} \right| = \left( { - 7} \right) - \left( { - 12} \right)\\
\Rightarrow \left| {x - 2} \right| = - 7 + 12\\
\Rightarrow \left| {x - 2} \right| = 5\\
\Rightarrow x - 2 = 5\,\text{hoặc}\,x - 2 = - 5\\
\Rightarrow x = 5 + 2\,\text{hoặc}\,x = - 5 + 2\\
\Rightarrow x = 7\,\text{hoặc}\,x = - 3\\
\text{Vậy}\,x = 7;x = - 3\\
b)150:\left( {x + 5} \right) = 2.5\\
\Rightarrow 150:\left( {x + 5} \right) = 10\\
\Rightarrow x + 5 = 150:10\\
\Rightarrow x + 5 = 15\\
\Rightarrow x = 15 - 5\\
\Rightarrow x = 10\\
\text{Vậy}\,x = 10\\
B2)\\
A = 3 + {3^2} + {3^3} + ... + {3^{2018}}\\
\Rightarrow 3.A = {3^2} + {3^3} + {3^4} + ... + {3^{2019}}\\
\Rightarrow 3A - A = {3^{2019}} - 3\\
\Rightarrow 2A = {3^{2019}} - 3\\
\Rightarrow A = \frac{{{3^{2019}} - 3}}{2}\\
Do:2A + 3 = {3^n}\\
\Rightarrow 2.\frac{{{3^{2019}} - 3}}{2} + 3 = {3^n}\\
\Rightarrow {3^{2019}} - 3 + 3 = {3^n}\\
\Rightarrow {3^{2019}} = {3^n}\\
\Rightarrow n = 2019\\
\text{Vậy}\,n = 2019
\end{array}$
