Is #f(x)=x^2+sin x# an even or odd function?
It is neither.
# = x^2-sinx#which is neither #f(x)#nor #-f(x)#
We cannot show that
but we can show that is fails to be true for all
It is, I think, clear that these numbers are neither equal nor negatives of each other.
An even function requires
An odd function requires
Our example satisfies neither of these conditions.