x to the power -1 is the same as 1/x, and as x tends to become infinitely large number, 1/x tends to become 0, hence it can simply be ignored in the denominator. Actually, that jump x to the power -1 made from denominator to numerator is a complete mathematical nonsense. Denominator should be factored and only then parts of such factored denominator could 'climb' over the fraction line (with its exponent changing sign) and multiply the numerator.