The process of the Heaviside Cover-up method works because the substitution replicates the results of Drsoccerball's generic substitution method.
Heaviside allows you to skip the step of common denominating
Re: HSC 2015 4U Marathon
Let x = \frac{1}{u}
\lim_{u \rightarrow \infty} \frac{1}{u} \text{ln}\left ( \frac{1}{u} \right ) = \lim_{u \rightarrow \infty} 0 - \frac{\text{ln}(u)}{u} = 0
As u approaches infinity, u increases more rapidly than ln(u), so the fraction ln(u)/u will tend to 0...