## Precalculus (10th Edition)

$z_1z_2=r_1r_2[\cos{(\theta_1+\theta_2)}+i\cdot \sin{(\theta_1+\theta_2)}]$.
$z_1z_2=r_1r_2[\cos{(\theta_1+\theta_2)}+i\cdot \sin{(\theta_1+\theta_2)}]$. Because the radiuses must be multiplied and the angles must be added.