Because n > m is, the same quadratic term can be reduced, and finally there are only unknowns on the molecule, which is equivalent to 1/n, and the limit is infinitely small.

For example: (x+x 2+x 3)/(x+2x 2+x 3+4x 4), the numerator and denominator are divided by x 3, (1/x2+1/x)/(1/x2+).

The same is true for N < m, only the other way around.