You shouldn’t use an article with school subjects, so we say:

I like math(s), geography, history etc.

My favourite subject is chemistry.

Instead, “the math” we may use in phrases referring to the math related area of a subject matter such as “a/the math competition” and “a/the math problem“.

Also, we use “the math” referring to some math calculations. For example: The math in this task seems to be taken from a high school textbook – I haven’t a clue how to do it.

Last but not least, it is widely used in the idiom “do the math“, which implies “put together all the facts to understand the truth, especially when it’s clear”.