Ta có: \(z = 3 - 4i \Rightarrow \overline z = 3 + 4i\).
Khi đó \(2z + \overline z = 2\left( {3 - 4i} \right) + 3 + 4i\) \( = 6 - 8i + 3 + 4i = 9 - 4i\).
\(\dfrac{{25i}}{z} = \dfrac{{25i}}{{3 - 4i}}\) \( = \dfrac{{25i\left( {3 + 4i} \right)}}{{\left( {3 - 4i} \right)\left( {3 + 4i} \right)}}\) \( = \dfrac{{75i - 100}}{{25}} = - 4 + 3i\)
