12.
\(log_3\left(log_2a\right)=0\Rightarrow log_2a=1\)
\(\Rightarrow a=2\)
13.
\(a=log_{2^3}.\left(15\right)^2=\dfrac{1}{3}.2.log_215=\dfrac{2}{3}log_215=\dfrac{2}{3}b\)
\(\Rightarrow3a=2b\)
14.
\(log_{49}28=log_{7^2}28=\dfrac{1}{2}log_728=\dfrac{1}{2}log_7\left(7.4\right)=\dfrac{1}{2}log_77+\dfrac{1}{2}log_74\)
\(=\dfrac{1}{2}+\dfrac{1}{2}log_72^2=\dfrac{1}{2}+log_72=\dfrac{1}{2}+m=\dfrac{1+2m}{2}\)
15.
\(a=\dfrac{1}{log_46}+\dfrac{1}{log_96}=log_64+log_69=log_636=2\)
\(\Rightarrow a\in\left\{\dfrac{1}{2};2;8\right\}\)
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