set theory:

6. Suppose a, b, c, and d are positive integers. Prove that (a) 2a 1 is odd. * (b) if a is even, then a + 1 is odd. if a is odd, then a + 1 is even. * (d) a(a + 1) is even. (e) 1 divides a and a divides a. * (f) if a divides b, then a sb