Đáp án:
$\begin{array}{l}
T = \frac{1}{{{5^{ - x - 1}}}} + 3.{\sqrt 5 ^{2x}} - {25^{\frac{{x - 1}}{2}}}\\
= {5^{x + 1}} + {3.5^x} - {5^{x - 1}}\\
= {5^x}.5 + {3.5^x} - \frac{1}{5}{.5^x}\\
= \frac{{39}}{5}{.5^x}\\
Khi\,{2^x} = \sqrt 7 \Rightarrow x = {\log _2}\sqrt 7 \\
\Rightarrow T = \frac{{39}}{5}{.5^{{{\log }_2}\sqrt 7 }} = 74,68
\end{array}$
