I was wondering in what circumstances are short final answers allowed. There are some case where I am left astonished of how such answers follow MSE's guidelines.
Generally answers should be "useful" and answer the question as asked. If the final result does answer the question as asked yet not as intended this is a problem with the question.
If a question asks for a derivation and the answer does not give one "it is not an answer" and can be flagged as such.
If however a final answer does answer the question as asked it is in principle fine as answer. If it is "useful" can then be a matter of opinion and expressed by votes.