Answer:
A quick hack is often to partially express some function in terms of a Taylor approximation about x0, since higher order terms of x go to zero if we are considering limits for (x−x0)→0. To really answer your question we need to know what the original question was, that is, about which point do you want the expansion? Let us assume around 0. Then we have the Maclaurin series:
cos(x)=1−12x2+O(x4)
You can add more terms if you need to. Now we write:
ln(1+(−12x2))=…
Do you know the standard Maclaurin series for this function?
Hint: it is of the form ln(1+u)
Explanation: