Skilgreinum $\text{cis}\theta = \cos \theta + i\sin\theta$. \\

Þá gildir $\text{cis}\theta\text{cis}\psi = \text{cis}(\theta + \psi)$, þ.e. ,,cis-fallið`` hefur veldisvísiseiginleikann $f(\theta + \psi) = f(\theta)f(\psi)$.\\

Því skilgreinum við (sjá nánar síðar)
\begin{center}
$$
e^{i\theta} = \cos \theta + i\sin \theta
$$
\end{center}
og ritum oft
$$
z = re^{i\theta}
$$
\underline{Ath:}
\begin{eqnarray*}
z_1z_2 &= r_1e^{i\theta_1}r_2e^{i\theta_2}\\
&= (r_1r_2)e^{i(\theta_1+ \theta_2)}
\end{eqnarray*}