I don't know what this "cis" function is.
So I would love if someone could please explain to be.
On Youtube, there is a video by Harold Walden proving that cis(a)*cis(b) = cis(a+b).
(sqrt(2)/6 * sqrt(6)/4) * (cis(3pi/5) * cis(2pi/3))
sqrt(12)/24 * cis(3pi/5 + 2pi/3)
sqrt(3)/12 * cis(19pi/15)