11. Find a proposition with three variables p, q, and r that is true when p and r are true and q is false, and false otherwise. 12. Find a proposition with three variables p, q, and r that is true when at most one of the three variables is true, and false otherwise. 13. Find a proposition with three variables p, q, and r that is never true. P 14. Find a proposition using only p, q, -, and the connective V T F with the truth table at the right. T F T F T T 15. Determine whether p - (q - r) and p - (q Ar) are equivalent. F F F. 16. Determine whether p - (q - r) is equivalent to (p -- q) - r. 17. Determine whether (p - 9) A (-p - 9) = 9