I'm explaining to a student that all functions can be divided into odd and even (symmetric and anti-symmetric) components. It is easy to prove (basic algebra or Taylor series) but is not referenced in the text. I'm using it in relation to the definition of trigonometric functions.
Does this theorem have a name? Are there any good online resources for this that cover this at a High School/Calc I level (i.e. no Fourier series). Or is this considered simply too obvious to cover formally?