Fill-in-the-blank proofs for Section 2.5

1. Claim: If an integer n has the form 3K + 1, then n does not have 9L + 5.
   Proof by contradiction. Click here.
   
2. Claim: For all real numbers x and y, either x ≥ (x + y)/2 or y ≥ (x + y)/2.
   Proof by contradiction. Click here.
   
3. Claim: For every integer n, if n² is even, then n is even.
   Proof by contradiction. Click here.
   
4. Claim: For every integer n, if n² is odd, then n is odd.
   Proof by contradiction. Click here.