Answer
$8\ cis(\frac{5\pi}{6})$
Work Step by Step
$if\\ $
$z_1=r_1(cosθ_1+isinθ_1) = r_1cis(\theta_1)\\$
$and\\$
$z_2=r_2(cosθ_2+isinθ_2)= r_2cis(\theta_2)\\ $
$then\\$
$z_1z_2=r_1r_2\ cis(θ_1+θ_2)]\\$
$given\ problem\\$
$2\ cis(\frac{2\pi}{3}).4\ cis(\frac{\pi}{6})\\$
$here\ r_1=2,r_2=4,θ_1=(\frac{2\pi}{3}),θ_2=(\frac{\pi}{6})\\$
$=2*4\ cis[(\frac{2\pi}{3})+(\frac{\pi}{6})]\\$
$=8\ cis(\frac{4\pi+\pi}{6})\\$
$=8\ cis(\frac{5\pi}{6})$
