Đáp án:
$\begin{array}{l}
c)x \ge 5\\
T = P + \frac{{10}}{x}\\
= 4x - 1 + \frac{{10}}{x}\\
= 4x + \frac{{10}}{x} - 1\\
Theo\,Co - si:4x + \frac{{10}}{x} \ge 2.\sqrt {4x.\frac{{10}}{x}} = 2.\sqrt {40} = 4\sqrt {10} \\
\Rightarrow T \ge 4\sqrt {10} - 1\\
\Rightarrow GTNN:T = 4\sqrt {10} - 1 \Leftrightarrow 4x = \frac{{10}}{x} \Rightarrow x = \frac{{\sqrt {10} }}{2}
\end{array}$
