please help 1 Prove using contraposition that for all intege

please help

1-  Prove using contraposition that for all integers m and n, if mn is even, then m is even or n is even.

2- Prove that for all integers a, b, c, s, and t, if a|b and a|c, then a|(sb ? tc).

3- Prove that for any even integer m and odd integer n that 3m + 5n is odd.

Solution

1. if both m and n are odd, then the product of odd with odd is always odd, To get even product atleast one has to be even.