## Precalculus: Mathematics for Calculus, 7th Edition

$z=2(\displaystyle \cos\frac{\pi}{2}+i\sin\frac{\pi}{2})$
See p. 604, A complex number $z=a+bi$ has the polar (or trigonometric) form $z=r(\cos\theta+i\sin\theta)$ where $r=|z|=\sqrt{a^{2}+b^{2}}$ and $\tan\theta=b/a$. The number $r$ is the modulus of $z$, and $\theta$ is an argument of $z$. ------- $z=0+2i$ $r=|z|=\sqrt{(0)^{2}+(2)^{2}}=2$ $\tan\theta$ is undefined, $\displaystyle \tan\frac{\pi}{2}$ is undefined. By symmetry (on the unit circle), $\theta$ can also be $\displaystyle \frac{3\pi}{2}$ (negative imaginary axis). $z=0+2i$ lies on the positive imaginary axis, so we select the appropriate argument: $z=2(\displaystyle \cos\frac{\pi}{2}+i\sin\frac{\pi}{2})$