2. Let A and B be sets. We can prove that A x B = B x A if and only if A = 0 or B = 0 or A = B. We have to prove two statements: (a) Prove this direction using contraposition: If A x B = B x A, then A=0 or B=0 or A = B. (b) Prove this direction directly: If A = 0 or B=0 or A = B, then A x B = Bx A. 3. Let x and y be positive real numbers. Prove by contradiction: If x - y = 1, then x or y (or both) are not integers.