An expression is termed as undefined (not to exist) if the denominator of the expression is zero. Given the expression a/a-b, the denominator of the expression is a-b. For the expression to be undefined, the expression at the denominator must be equal to zero i.e a-b = 0
If a - b = 0, then a = 0+b; a = b
Another case for the function not to exist is if a = b
