Đáp án:
\(\begin{array}{l}
C1:B\\
C2:C\\
C3:C\\
C4:D\\
C5:A\\
C6:B
\end{array}\)
Giải thích các bước giải:
\(\begin{array}{l}
C1:\\
M = \dfrac{1}{2}\sqrt 5 - 3.2\sqrt 5 + \dfrac{1}{3}.3\sqrt 5 \\
= \left( {\dfrac{1}{2} - 6 + 1} \right)\sqrt 5 \\
= - \dfrac{9}{2}\sqrt 5 \\
\to B\\
C2:\\
\sqrt {{5^2}} + \sqrt {{{\left( { - 5} \right)}^2}} = 5 + 5 = 10\\
\to C\\
C3:\\
\sqrt {{a^2}} = - a \to \left| a \right| = - a\\
\to - a = - a\left( {DK:a \le 0} \right)\\
\to C\\
C4:\\
\dfrac{6}{{\sqrt 7 - 1}} = \dfrac{{6\left( {\sqrt 7 + 1} \right)}}{{7 - 1}} = \sqrt 7 + 1\\
\to D\\
C5:\\
\dfrac{y}{x}.\sqrt {\dfrac{{{x^2}}}{{{y^4}}}} = \dfrac{y}{x}.\dfrac{{\left| x \right|}}{{{y^2}}}\\
= \dfrac{y}{x}.\dfrac{x}{{{y^2}}} = \dfrac{1}{y}\\
\to A\\
C6:\sqrt {81} = \sqrt {{9^2}} = 9\\
\to B
\end{array}\)
