2. 3 a. Prove by contraposition: If 20 - 2 is irrational , then a is irrational. Hint: Show that if a is rational, then 20 - 5 is rational. 3 b. Use the fact that , 11 is irrational to prove by contradiction that 31I - 1 5 is irrational. Answer: Assume that 3,11 - 1 5 is rational. Derive a contradiction to the fact that 11 is irrational