first: i have to assume here that

Suppose .

1) if , then and . combining the two facts, we have for .

2) if , then and . combining the two facts again, we have for a<0.

3) if a=0, then it is trivial.

Thus, for any ,

next:

Suppose

1) if ,

2) if , use the fact that . thus,

3) if , trivial.