## Precalculus (6th Edition) Blitzer

$$z=5i$$
$$5 \left ( \cos \frac{\pi }{2}+i \sin \frac{\pi }{2} \right )= 5 (0+i)=5i$$So the rectangular form of the complex number $5 \left ( \cos \frac{\pi }{2}+i \sin \frac{\pi }{2} \right )$ is$$z=5i.$$