Let's say "ab" means the product of two numbers, how would you choose the values of a and b to construct a square with sides of length ab?

Using line AB, you can construct a square using that line. You first join the first two lines, and then you call the second point A1B1.then join the square with the third line and call it line A2B2, and then complete the square with point A3B3. By joining these points you will have constructed your square.