67.
\(log40=\dfrac{log_240}{log_210}=\dfrac{log_2\left(2^3.5\right)}{log_2\left(2.5\right)}=\dfrac{3+log_25}{1+log_25}=\dfrac{a+3}{a+1}\)
68.
\(log_23=\dfrac{log_53}{log_52}=log_53.log_25=ab\)
\(\Rightarrow log_{60}75=\dfrac{log_275}{log_260}=\dfrac{log_2\left(3.5^2\right)}{log_2\left(2^2.3.5\right)}=\dfrac{log_23+2log_25}{2+log_23+log_25}\)
\(=\dfrac{ab+2a}{2+ab+a}\)
70.
\(log_32=\dfrac{log_52}{log_53}=\dfrac{log_35}{log_25}=\dfrac{b}{a}\)
\(log_{140}98=\dfrac{log_398}{log_3140}=\dfrac{log_3\left(2.7^2\right)}{log_3\left(2^2.5.7\right)}=\dfrac{log_32+2log_37}{2log_32+log_35+log_37}=\dfrac{\dfrac{b}{a}+\dfrac{2}{c}}{\dfrac{2b}{a}+b+\dfrac{1}{c}}\)
\(=\dfrac{bc+2a}{abc+2bc+a}\)
. Chọn đáp án sai:
