we can prove the claim by showing:
(1) and (2)
(1) is immediate.
for (2): ....there should be a theorem in your book telling you something to that effect
hint: is the cardinal number of by definition. you can prove that this cardinal number is also infinitely countable, which will show that it is equal to . the way i usually see this done is by the use of a diagram, you should also see a proof using this technique in your text. the diagram looks like a grid, with the elements of along the first row and column, and the elements in the inner cells are there corresponding products. you count off the elements in diagonals ...or
Xo * Xo = Xo