what is the difference between O(nk) and O(n+k) in time complexity?

Question

In big O notation of time complexity in algorithmic analysis, when an algorithm depends on n and k, what is the difference between these two notations. Also pls help in the

Answer 1

It should be clear that these are different since, for example if n=k:

O(nk)=O(nn)=O(n^2)

O(n+k)=O(2n)=O(n)