Đáp án:
$\begin{array}{l}
B4)\\
a)82 - \left( {15 + x} \right) = 72\\
\Leftrightarrow 82 - 15 - x = 72\\
\Leftrightarrow x = 82 - 15 - 72\\
\Leftrightarrow x = - 5\\
Vậy\,x = - 5\\
b)17 - \left( {43 - \left| {x + 1} \right|} \right) = 45\\
\Leftrightarrow 17 - 43 + \left| {x + 1} \right| = 45\\
\Leftrightarrow \left| {x + 1} \right| = 45 + 43 - 17\\
\Leftrightarrow \left| {x + 1} \right| = 71\\
\Leftrightarrow \left[ \begin{array}{l}
x + 1 = 71 \Leftrightarrow x = 70\\
x + 1 = - 71 \Leftrightarrow x = - 72
\end{array} \right.\\
Vậy\,x = 70;x = - 72\\
c){3^2}.x = - 27\\
\Leftrightarrow x = - 27:9\\
\Leftrightarrow x = - 3\\
Vậy\,x = - 3\\
d)x + 15 = - 2\\
\Leftrightarrow x = - 2 - 15\\
\Leftrightarrow x = - 17\\
Vậy\,x = - 17\\
e)3x - 2 = - 17\\
\Leftrightarrow 3x = - 17 + 2\\
\Leftrightarrow 3x = - 15\\
\Leftrightarrow x = - 15:3\\
\Leftrightarrow x = - 5\\
Vậy\,x = - 5\\
f)9 - 25 = \left( {7 - x} \right) - \left( {25 + 7} \right)\\
\Leftrightarrow - 16 = 7 - x - 32\\
\Leftrightarrow x = 7 - 32 + 16\\
\Leftrightarrow x = - 9\\
Vậy\,x = - 9\\
B5)\left\{ \begin{array}{l}
21 \vdots x\\
x \vdots \left( { - 7} \right)
\end{array} \right.\\
\Leftrightarrow \left\{ \begin{array}{l}
x \in \left\{ { - 21; - 7; - 3; - 1;1;3;7;21} \right\}\\
x \in \left\{ { - 7; - 14; - 21; - 42;...;7;14;21;42;...} \right\}
\end{array} \right.\\
\Leftrightarrow x \in \left\{ { - 21; - 7;7;21} \right\}\\
Vậy\,x \in \left\{ { - 21; - 7;7;21} \right\}
\end{array}$
