The pair of numbers whose difference is 18 and whose product is as small as possible is? The minimum product is?

Assume the bigger number is x, since the difference is 18, another one should be x - 18

So the product will be x (x-18) = x^2 - 18x = (x-9) ^2 - 81

Since the minimum of (x-9) ^2 is 0, then the minimum of the product is - 81