A set is called countable if it is finite or denumerable.
For instance, The set of all natural numbers is countable, as we could count all its elements in the way 0,1,2,3,...
The set of all integers is also countable, as we could count all its elements in the way 0,1,-1,2,-2, ...
a) Show that the set of all potitive rational numbers is countable.
b) Hence, show that the set of all rational numbers is countable.
c) Show that the set of all irrational numbers is NOT countable.​