On a TI-83/89 scientific calculator, pressing the 'MATH' button and then the '9' button prompts a function that enables the user to find the answer to a type of problem called a definite integral within a few keystrokes. The alternative to this is doing it all by hand, which is very time-consuming/tedious. So, by simply using "MATH-9" you save yourself some time and a headache. This can be loosely applied to situations in life that require work, but whoever is supposed to be doing that work is lazy/uninterested so they opt out of it by finding a simple alternative.
Bob: Ugh, my girlfriend was bugging me to mow the lawn all day!
Charlie: So did you give in or what?
Bob: Nah, I just MATH-9'd that shit and paid some neighborhood kid to do it.
When a Maths question tries to trick you/when you think you finished your maths work but there is more to do
*John* Ok let's do this maths question. *sees the question and gets confused* huh whats this ahh its trying to math-bait me.
*Student* Finally I'm done! Miss I'm finished! *Teacher comes and checks his work* sees that he has one left* *Teacher* Actually you have not. *Shows him the extra sheet* *Student screams*