(a) Prove that the following statement is true for every natural number n, by using a sequence of equivalences: (3n+2)? > 12n + 13. 4] (b) (i) Use proof by contraposition to prove that the following statement is true for all integers m and n: If mn is even, then m is even or n is even. (3] (ii) Hence prove that the following statement is true for all integers m and n: mn is even if and only if m is even or n is even. 3]