Prove the following using proof by contrapositive, proof by contradiction or an existence proof (a) if the gcd(a, b)1, then at most one of a or b is even (b) there are no positive real roots of 2 4x +5 (c) if r and s are both positive real numbers, then + st vf+ vs. (d) the only three consecutive odd natural numbers that are all prime are 3, 5, and (e) v5 is not a rational number (0) if r is a rectangle of area a, then there exists a square with the same area.