I think most true geeks would have looked it themselves before getting to comments, too?
Are you new to
...why can airlines require credit card payment and refuse my cash if I wanted to pay for their overpriced stuff in the air?
Well, technically, and since this is \. I can be technical, the airlines are not refusing to take Dollars. They are just unable to process paper and coin transactions. Just like they could say you can't use American Express to pay, if they found that the business cost of doing so is more than the business benefit of being able to access those cards.
In any formula, constants (especially those obtained from handbooks) are to be treated as variables.