Câu hỏi 48

\(\begin{align}  & \,\,\,\,\,\left( 3+i \right)\left| z \right|=\frac{-2+14i}{z}+1-3i \\  & \Leftrightarrow \left( 3+i \right)\left| z \right|-1+3i=\frac{-2+14i}{z} \\  & \Leftrightarrow \left( 3\left| z \right|-1 \right)+\left( \left| z \right|+3 \right)i=\frac{-2+14i}{z} \\ \end{align}\)

Lấy mođun hai vế ta có : \(\sqrt{9{{\left| z \right|}^{2}}-6\left| z \right|+1+{{\left| z \right|}^{2}}+6\left| z \right|+9}=\frac{10\sqrt{2}}{\left| z \right|}\)

\( \Leftrightarrow 10{\left| z \right|^2} + 10 = \frac{{200}}{{{{\left| z \right|}^2}}} \Leftrightarrow {\left| z \right|^4} + {\left| z \right|^2} - 20 = 0 \Leftrightarrow {\left| z \right|^2} = 4 \Rightarrow \left| z \right| = 2 \in \left( {\frac{7}{4};\frac{{11}}{5}} \right)\)

Chọn D.