I use them pretty interchangeably. To me the "math" as a singular seems weird except as a verb 😅 like each equation has multiple terms in the expression so doing math with maths.
My wife is my best friend.
We talk mad funny shit to each other, as best friends do.
I would never, however bad mouth my wife, who is the mother of my children, to other people, let alone the internet. Some of y'all are fucked.