"Sir, is there any videos on trig identities up or notes? I can't really follow the one you just put up under the Demoivres theorem notes?"
I do have notes and further examples of Trigonometric identities and deMoivre in the Complex Number page, I have re linked them here. The example with cos(3x) is probably the most important to follow since it's mentioned in the syllabus.
- Examples from Class Notes in Cornell Style (click here)
Below are 3 Examples from Mathsireland.com
If you would like a video explanation of any of these examples just ask.