18. if the product of two integers is odd, then both must be odd integers. prove by contradiction, where p is prime number. a 19. 2 is an irrational number. 20. 5 is an irrational number. 21. p is an irrational number. *22. log102 is an irrational number. prove by cases, where n is an arbitrary integer and x denotes the absolute value of x.