If the fractions are complicated, there are ways to simplify them.
In many cases, we compute a sum or difference of fractions by finding
the common denominator. For instance,
$$\frac{1}{4} + \frac{1}{x} = \frac{x}{4x} + \frac{4}{4x} = \frac{x+4}{4x},$$
and
$$\frac{1}{t} - \frac{1}{t^2+t} = \frac{t+1}{t^2+t}-\frac{1}{t^2+t}
= \frac{t}{t^2+t} = \frac{1}{t+1}.$$
The following video shows how to use these sorts of manipulations to help
evaluate limits.