Câu hỏi 37

Theo bài ra ta có:

\(\begin{array}{l}\left( {z + \dfrac{5}{{\left| z \right|}}} \right)i = 7 - z \Leftrightarrow zi + \dfrac{{5i}}{{\left| z \right|}} = 7 - z \Leftrightarrow z\left( {i + 1} \right) = 7 - \dfrac{{5i}}{{\left| z \right|}}\\ \Leftrightarrow 2{\left| z \right|^2} = 49 + \dfrac{{25}}{{{{\left| z \right|}^2}}} \Leftrightarrow 2{\left| z \right|^4} - 49{\left| z \right|^2} - 25 = 0\\ \Leftrightarrow \left[ \begin{array}{l}{\left| z \right|^2} = 25\,\,\left( {tm} \right)\\\left| z \right| =  - \dfrac{1}{2}\,\,\left( {ktm} \right)\end{array} \right. \Leftrightarrow \left| z \right| = 5\,\,\left( {Do\,\,\left| z \right| > 0} \right)\end{array}\)

Thay \(\left| z \right| = 5\) vào biểu thức đề bài ta có:

\(\left( {z + 1} \right)i = 7 - z \Leftrightarrow z\left( {i + 1} \right) = 7 - i \Leftrightarrow z = \dfrac{{7 - i}}{{i + 1}} = 3 - 4i\).

Chọn C.