\(\cis\) 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}