I am having trouble understanding the application to this problem:
I think this means the number of combinations of two numbers from the sets $A_1$ and $A_2$?
Now Alessandro Zaccagnini goes on to show the solution (below) and I do not understand why $A$ x $A$ in $(a_1,a_2)\,\epsilon\, A$ x $A$? Why would the sets be multiplied?
