I saw on a video that when finding the limit as n approaches infinity of 1/(n+2), the top and bottom of the fraction is multiplied by 1/n. Similarly, for the limit as n approaches infinity of 4n/sqrt(n^2+5), the top and bottom are multiplied by 1/n, except sqrt(n^2+5) is multiplied by sqrt(1/n^2) which is the same as 1/n. I want to know, why is this done? I don't get why he does this, is there a specific law that allows it?