trivial

used in mathematics for proofs that are considered easy after understanding them

student: i don't understand how this is obvious.
prof: hmm let me get back to you tomorrow, i don't remember how to prove it right now.

1 week later:
student: about that proof...
prof: ah yes, i just finished reading the proof but don't worry. it is indeed trivial.

trivial

used in mathematics for proofs that are considered easy after understanding them

student: i don't understand how this is obvious.
prof: hmm let me get back to you tomorrow, i don't remember how to prove it right now.

1 week later:
student: about that proof...
prof: ah yes, i just finished reading the proof but don't worry. it is indeed trivial.