The problem is that formula_4 could be any number. It would work if formula_4 were 1 or if it were 1,000,000,000. 0/0 is said to be of "indeterminate form" for this reason, because it has no single value. Numbers of the form A/0, on the other hand, where formula_4 is not 0, are said to be "undefined", or "undeterminated." This is because any attempt to define them will result in a value of infinity, which is itself undefined. Usually when two numbers are equal to the same thing, they are equal to each other. That is not true when the thing they are both equal to is 0/0. This means that the normal rules of maths do not work when the number is divided by zero.