A set S is said to be closed under subtraction if:
[tex]\forall a,b\in S,\text{ a-b}\in S\text{ and b-a}\in S[/tex]
Since the set given is that of positive numbers, we pick two different positive numbers, say 6 and 9.
[tex]\begin{gathered} \\ 9-6=3\in S \\ 6-9=-3\notin S \end{gathered}[/tex]
Since -3 is not a positive number, we can then conclude that the set of positive numbers are
