Ta có \(\overline{z}(1+2i)=7+4i\Leftrightarrow \overline{z}=\frac{7+4i}{1+2i}\) \(\Leftrightarrow \overline{z}=\frac{(7+4i)(1-2i)}{(1+2i)(1-2i)}\) \(\Leftrightarrow \overline{z}=\frac{7-14i+4i-8i^2}{1-4i^2}\Leftrightarrow \overline{z}=\frac{15-10i}{5}=3-2i\) Suy ra z = 3 + 2i Do đó w = z + 2i = 3 + 4i Vậy \(\left | w \right |=\sqrt{3^2+4^2}=5\)