Câu 13:
\(\log_{\dfrac{a}{7}}\left(\dfrac{a^3}{343}\right)\)
\(=\log_{\dfrac{a}{7}}\left(\dfrac{a}{7}\right)^3\)
=3
=>Chọn B
Câu 14:
\(4^{log_2\sqrt{3}}=2^{log_23}=3\)
=>Chọn B
Câu 15:
\(\log_{a^3}\left(\dfrac{a^5}{\sqrt[4]{b}}\right)=2\)
=>\(\log_{a^3}a^5-\log_{a^3}b^{\dfrac{1}{4}}=2\)
=>\(\dfrac{1}{3}\cdot5-\dfrac{1}{3}\cdot\dfrac{1}{4}\cdot\log_ab=2\)
=>\(\dfrac{1}{12}\cdot\log_ab=\dfrac{5}{3}-2=-\dfrac{1}{3}\)
=>\(\log_ab=-\dfrac{1}{3}:\dfrac{1}{12}=-4\)
=>Chọn D
Câu 18:
\(P=\log_ab^3\cdot\log_ba^4\)
\(=3\cdot\log_ab\cdot4\cdot\log_ba\)
=12
=>Chọn C
Câu 19: \(log_{\dfrac{1}{3}}\left(\dfrac{1}{3}\right)=1;log_{\dfrac{1}{3}}3=log_{\dfrac{1}{3}}\left(\dfrac{1}{3}\right)^{-1}=-1;log_{\dfrac{1}{2}}8=log_{\dfrac{1}{2}}\left(\dfrac{1}{2}\right)^{-3}=-3\)
=>Loại A,B,D
=>Chọn C
Câu 20:
\(\log_3\Omega\simeq\log_33,14>1\)
=>Loại A
\(log_35>1>\log_53\)
=>Chọn C
. Chọn đáp án sai:
