G6.1 If $\log _{2} a-2 \log _{a} 2=1$, find $a$.
$\begin{array}{l}\frac{\log a}{\log 2}-\frac{2 \log 2}{\log a}=1 \\ (\log a)^{2}-2(\log 2)^{2}=\log 2 \log a \\ (\log a)^{2}-\log 2 \log a-2(\log 2)^{2}=0 \\ (\log a-2 \log 2)(\log a+\log 2)=0 \\ \log a=2 \log 2 \text { or }-\log 2 \\ a=4 \text { or } \frac{1}{2}\end{array}$