understanding, comprehension / appreciation

understand, comprehend / appreciate

needed, necessary

need, necessity

mathematics (field of study)

question, problem on homework etc. / a question; a matter (of time etc.) / a problem (something that needs to be dealt with); a trouble / an issue; a matter

(mathematics, computing) function

impossible to understand, utterly confused

necessity (quality or state of being necessary, unavoidable, or absolutely requisite)

unnecessary