`D = (0; +infty) \\ {2}`
`log_{2} (4x) - `$log_{\dfrac{x}{2}} 2$` = 3`
`-> log_{2} 4 + log_{2} x -`$ \dfrac{1}{log_{2} \dfrac{x}{2}}$` = 3`
`-> 2 + log_{2} x - 1/(log_{2} x - log_{2} 2) = 3`
`-> log_{2} x - 1/(log_{2} x - 1) = 1` `(1)`
`text{Đặt}` `log_{2} x = t`
`(1)`
`-> t - 1/(t - 1) = 1`
`-> t(t - 1) - 1 - (t - 1) = 0`
`-> t^2 - t - 1 - t + 1 = 0`
`-> t^2 - 2t = 0`
`-> t(t - 2) = 0`
`->` \(\left[ \begin{array}{l}t = 0\\t = 2\end{array} \right.\) `(TM)`
`->` \(\left\{ \begin{array}{l}log_{2} x = 0\\log_{2} x =2\end{array} \right.\)
`->` \(\left\{ \begin{array}{l}x=1\\x=4\end{array} \right.\)
`-> S = {1; 4}`
`->\ text{Chọn B}`
