Question

why is the sign for the sum and difference formula for cos(u+/-v) = cosucosv-/+sinusinv i.e. the sign is negative instead of positive

Answer 1

like in calulus when you want to find the derivative of cosx using first principle and the identity is cosucosv-sinusinv

Answer 2

Are you familiar with Euler's Formula?
\[e^{i\theta} = \cos(\theta)+isin(\theta)\]
There is a pretty good proof using that fact that im uploading in a sec

Answer 3

Here ya go :)

Answer 4

if theres something i put that isnt clear, feel free to ask so i can clarify :)

Answer 5

no am not familiar with euler's

Answer 6

i dont understand lol...

Answer 7

thanks though joe!

Answer 8

Good luck with that one...the other proof isn't all that great as well.