10. Given a, b E Z, prove that if a²(b² - 2b) is odd, then both a and b are odd.
11. Given m, n = Z, prove that if 25 + mn, then 5 m or 5 tn.
12. If m, n E Z and m + n is even, prove that m² + n² is even.
13. Prove that an integer n is even if and only if n² + 2n + 9 is odd.
14. If a is an odd integer, prove that a² + 3a + 5 is odd.