Đáp án:12 B
13 D
14B
15B
Giải thích các bước giải
\(\begin{array}{l}
12,{(\frac{2}{{\sqrt 5 }})^{\frac{1}{x}}} \le {(\frac{2}{{\sqrt 5 }})^3}\\
\Rightarrow \frac{1}{x} \ge 3 \Leftrightarrow 0 < x \le \frac{1}{3}\\
chonB\\
{13,4^x}{.3^3} > {3^x}{.4^3}\\
\Leftrightarrow {\frac{4}{3}^x} > {\frac{4}{3}^3}\\
\Leftrightarrow x > 3\\
\Leftrightarrow x = 4\\
chonD\\
{14,8^x}{.2^{1 - {x^2}}} > {(\sqrt 2 )^{2x}}\\
\Leftrightarrow {2^{3x}}{.2^{1 - {x^2}}} > {2^x}\\
\Leftrightarrow 3x + 1 - {x^2} > x\\
\Leftrightarrow {x^2} - 2x - 1 < 0\\
\Leftrightarrow 1 - \sqrt 2 < x < 1 + \sqrt 2 \\
\Rightarrow x = 0,1,2\\
chonB\\
{15,3^{1 - x}} + {2.3^x} \le 7\\
\Leftrightarrow \frac{3}{{{3^x}}} + {2.3^x} \le 7\\
a = {3^x}(a > 0)\\
\Rightarrow 3 + 2{a^2} - 7a \le 0\\
\Leftrightarrow (a - 3)(a - \frac{1}{2}) \le 0\\
\Leftrightarrow \frac{1}{2} \le a \le 3\\
\Leftrightarrow x = 0,1\\
chonB
\end{array}\)
