Answer
$z_1z_2=-8$
$\frac{z_1}{z_2}=\sqrt 3+i$
Work Step by Step
Apply the formulas from page 605:
$z_1z_2=4·2[cos(\frac{7\pi}{12}+\frac{5\pi}{12})+i~sin(\frac{7\pi}{12}+\frac{5\pi}{12})]$
$z_1z_2=8[cos~\pi+i~sin~\pi]$
$z_1z_2=8(-1+i0)$
$z_1z_2=-8$
$\frac{z_1}{z_2}=\frac{4}{2}[cos(\frac{7\pi}{12}-\frac{5\pi}{12})+i~sin(\frac{7\pi}{12}-\frac{5\pi}{12})]$
$\frac{z_1}{z_2}=2[cos(\frac{\pi}{6})+i~sin(\frac{\pi}{6})]$
$\frac{z_1}{z_2}=2[\frac{\sqrt 3}{2}+i(\frac{1}{2})]$
$\frac{z_1}{z_2}=\sqrt 3+i$
