[tex]\dfrac{5+12i}{3+4i}=\dfrac{5+12i}{3+4i}\cdot\dfrac{3-4i}{3-4i}=\dfrac{15-20i+36i-49i^2}{3^2-(4i)^2}\\\\=\dfrac{15+16i+49}{9-16i^2}=\dfrac{64+16i}{9+16}=\dfrac{64+16i}{25}=2.56+0.64i[/tex]
[tex]z=a+bi\\\\|z|=\sqrt{a^2+b^2}\\\\z=2.56+0.64i\to a=2.56;\ b=0.64\\\\|z|=\sqrt{2.56^2+0.64^2}=\sqrt{6.9632}=\sqrt{\dfrac{69632}{10000}}=\sqrt{\dfrac{4096\cdot17}{100}}\\\\=\dfrac{64\sqrt{17}}{100}=0.64\sqrt{17}\approx2.6=2\dfrac{3}{5}[/tex]
