Prove that for all integers $a,b$ and $c,$ if $a|b||$ and $ac$ then $a(b+c).$

Prove each of the statements in two ways: $($a$)$ by contraposition and $($b$)$ by contradiction.

$3.$ For every integer $n,$ if $n^2$ is odd then $n$ is odd.

$4.$ For all integers $a,b$ and $c,$ if $abc$ then $ab$