\(\cis\) notation of a complex number, \(z\)
\[\boxed{\|z\|\cdot \cis(\theta) = \|z\|\cdot\left[\cos(\theta)+i\sin(\theta)\right]}\]
Euler's formula
\[\boxed{\|z\|\cdot e^{i\theta} = \|z\|\cdot\left[\cos(\theta)+i\sin(\theta)\right]}\]
My real issue with \(\cis\) notation is that it's not as obvious what certain operations do. You can't use previous knowledge acquired from index laws. By using \(\cis\), you are restricting yourself and you lose a lot of the complexity (Haha get it? complex? okay...) that is possible with the polar form.
Question 1: Express \(i^i\) in the form \(\alpha+\beta i\)
For any method, the first step is to turn this into polar form.
\begin{align}
\text{Define }z&=i\\
\implies&{\|z\| = 1}\\
\implies&{\theta = \frac{\pi}{2}}
\end{align}
Now we want to evaluate \(z^z\)
Let's try get somewhere with \(\cis\).
\begin{align}
z &= 1\cdot\cis\left(\frac{\pi}{2}\right)\\
&= \cis\left(\frac{\pi}{2}\right)\\
z^z &= \cis\left(\frac{\pi}{2}\right)^{\cis\left(\frac{\pi}{2}\right)}\\
&= \cis\left(\frac{\pi}{2}\right)^i\\
&= \cis\left(i\frac{\pi}{2}\right)\\
&= ?
\end{align}
Now what? We have an \(i\) in the phase.
It would be much easier if this was a complex exponential!
\begin{align}
z &= 1\cdot e^{i\frac{\pi}{2}}\\
&= e^{i\frac{\pi}{2}}\\
z^z &= {\left[e^{i\frac{\pi}{2}}\right]}^{e^{i\frac{\pi}{2}}}\\
&= {\left[e^{i\frac{\pi}{2}}\right]}^{i}\\
&= {\left[e^{i\cdot i\frac{\pi}{2}}\right]}\\
&= e^{-\frac{\pi}{2}}
\end{align}
Wow! And it's neatly packaged as an exponent!
I understand the use of \(\cis\) to represent a complex number as a polar coordinate if your curriculum doesn't understand the concept of calculus when the concept of complex numbers is being taught
Bonus Math Tip: Derive the double angle identities.
No need for Euler's formula here, use \(\cis\) for this trick if you want.
Not sure why you'd need to know this - sure, in methods these identities aren't given on the formula sheet but they are in specialist. Methods seems to stick to the basic identities such as the 2As and pythagorean, but you're expected to remember them. I always just put these identities on my notes.
Use this trick to derive any set of angle identities (triple, quadruple, etc.). Have fun expanding brackets though.
Let the first angle be \(A\) and the second angle \(B\).
\begin{align}
e^{\pi A}\cdot e^{\pi B} &= \left[\cos(A)+i\sin(A)\right]\cdot\left[\cos(B)+i\sin(B)\right]\\
e^{\pi (A+B)} &= \cos(A)\cos(B) + i\cos(A)\sin(B) + i\sin(A)\cos(B) + i^2\sin(A)\sin(B)\\
\cos(A+B) + i\sin(A+B) &= \left[\cos(A)\cos(B) - \sin(A)\sin(B)\right] + i\cdot\left[\cos(A)\sin(B) + \sin(A)\cos(B) \right]
\end{align}
Consider the real and complex components of this equation to get the two identities.
\begin{cases}
\text{Real:}&\cos(A+B) = \cos(A)\cos(B) - \sin(A)\sin(B)\\
\text{Imaginary:}&\sin(A+B) = \cos(A)\sin(B) + \sin(A)\cos(B)
\end{cases}
And you can repeat this all again for negative second angle.
\begin{align}
e^{\pi A}\cdot e^{\pi (-B)} &= \left[\cos(A)+i\sin(A)\right]\cdot\left[\cos(-B)+i\sin(-B)\right]\\
&= \left[\cos(A)+i\sin(A)\right]\cdot\left[\cos(B)-i\sin(B)\right]\\
e^{\pi (A-B)} &= \cos(A)\cos(B)-i\cos(A)\sin(B)+i\sin(A)\cos(B)-i^2\sin(A)\sin(B)\\
\cos(A-B) + i\sin(A-B) &= \left[\cos(A)\cos(B) + \sin(A)\sin(B)\right] + i\cdot\left[\sin(A)\cos(B)-\cos(A)\sin(B)\right]
\end{align}
\begin{cases}
\text{Real:}&\cos(A-B) = \cos(A)\cos(B) + \sin(A)\sin(B)\\
\text{Imaginary:}&\sin(A-B) = \sin(A)\cos(B)-\cos(A)\sin(B)
\end{cases}