12: D( f ) = ℝ,
this is a symmetric set.
Using the fact that sine is odd we get
![](gif2/ecb2al2.gif)
This does not look like f (x) nor like
-f (x). To
be sure of this we try to solve the equations
f (−x) = f (x)
and
f (−x) = −f (x) and we see that they are
not true for all x.
Answer