Đáp án:
c) x=2
Giải thích các bước giải:
\(\begin{array}{l}
c)\dfrac{{1800}}{{{3^x} - 14 + 30}} = 72\\
\to {3^x} + 16 = \dfrac{{1800}}{{72}}\\
\to {3^x} + 16 = 25\\
\to {3^x} = 9\\
\to {3^x} = {3^2}\\
\to x = 2\\
d)2x + 1 \in U\left( {x + 5} \right)\\
\Leftrightarrow x + 5 \vdots 2x + 1\\
\to 2x + 10 \vdots 2x + 1\\
\to 2x + 1 + 9 \vdots 2x + 1\\
\to 9 \vdots 2x + 1\\
\to 2x + 1 \in U\left( 9 \right)\\
\to \left[ \begin{array}{l}
2x + 1 = 9\\
2x + 1 = 3\\
2x + 1 = 1
\end{array} \right.\\
\to \left[ \begin{array}{l}
x = 4\\
x = 1\\
x = 0
\end{array} \right.
\end{array}\)
