For the addition, can't you just construct it? A sum of an odd number of odd whole numbers is odd...
For a > 0, a + (a+2) + (a+4) + (a+(n-1)*2) -- There. An odd number of odd Whole Numbers
Then n(a) + 2 + 4 + ... + (n-1)*2 -- Oh, well that's pretty silly. Everything but "n(a)" clearly is EVEN.
What about n(a)? Prove it! Remember, both n and a are odd!