3.51. Let x ∈ Z. Use one or two lemmas to prove that if x is odd, then x² is even.
3.52. Give a direct proof of the following: Let x ∈ Z. If 11x – 5 is odd, then x is even.
3.53. Let x, y ∈ Z. Prove that if x + y is odd, then x and y are of opposite parity.