Bài 1:

\(A=\log_380=\log_3(2^4.5)=\log_3(2^4)+\log_3(5)\)

\(=4\log_32+\log_35=4a+b\)

\(B=\log_3(37,5)=\log_3(2^{-1}.75)=\log_3(2^{-1}.3.5^2)\)

\(=\log_3(2^{-1})+\log_33+\log_3(5^2)=-\log_32+1+2\log_35\)

\(=-a+1+2b\)

Bài 2:

\(\log_{30}8=\frac{\log 8}{\log 30}=\frac{\log (2^3)}{\log (10.3)}=\frac{3\log2}{\log 10+\log 3}\)

\(=\frac{3\log (\frac{10}{5})}{1+\log 3}=\frac{3(\log 10-\log 5)}{1+\log 3}=\frac{3(1-b)}{1+a}\)

Bài 3:

\(\log_{27}5=a; \log_87=b; \log_23=c\)

\(\Leftrightarrow \frac{\ln 5}{\ln 27}=a; \frac{\ln 7}{\ln 8}=b; \frac{\ln 3}{\ln 2}=c\)

\(\Leftrightarrow \frac{\ln 5}{\ln (3^3)}=a; \frac{\ln 7}{\ln (2^3)}=b; \ln 3=c\ln 2\)

\(\Leftrightarrow \frac{\ln 5}{3\ln 3}=a; \frac{\ln 7}{3\ln 2}=b; \ln 3=c\ln 2\)

\(\Rightarrow \frac{\ln 5}{3c\ln 2}=a; \frac{\ln 7}{3\ln 2}=b\)

\(\Rightarrow \ln 35=\ln 5+\ln 7=3ac\ln 2+3b\ln 2\)

Do đó:
\(D=\log_6 35=\frac{\ln 35}{\ln 6}=\frac{\ln 35}{\ln 2+\ln 3}=\frac{\ln 35}{\ln 2+c\ln 2}=\frac{3ac\ln 2+3b\ln 2}{\ln 2+c\ln 2}\)

\(=\frac{3ac+3b}{1+c}\)

\(log_{\sqrt{10}}30=\dfrac{log_230}{log_2\sqrt{10}}=\dfrac{log_22+log_23+log_25}{\dfrac{1}{2}\left(log_22+log_25\right)}=\dfrac{2\left(1+a+b\right)}{1+b}\)

Chọn B

Đáp án C.

a: \(8^{log_25}=2^{3\cdot log_25}=5^3=125\)

b: \(\left(\dfrac{1}{10}\right)^{log81}=10^{-1\cdot log81}=81^{-1}=\dfrac{1}{81}\)

c: \(5^{log_{25}16}=5^{log_{5^2}16}=16^{-2}=\dfrac{1}{256}\)

\(log_a\left(a^3b^2\right)=log_aa^3+log_ab^2=3+2\cdot log_ab\)

=>B

a) \(\log_381=\log_33^4=4\log_33=4.1=4\)

b) \(\log_{10}\dfrac{1}{100}=\log_{10}10^{-2}=-2\log_{10}10=-2.1=-2\)

a: \(log_381=4\)

b: \(log_{10}\left(\dfrac{1}{100}\right)=-2\)