Prove by dirrect or contrapositive method If x and y are two

Prove by dirrect or contrapositive method.....

If x and y are two integers whose product is even, then at least one of the two must be even.

if x and y are two integers whose product is odd, then both must be odd.

for any integer k, if k is even, 3k+1 is odd.

if a and b a real numbers such that the product ab is an irrational number, then either a or b must be an irrational number.

Solution