Modern mathematical symbols originate several hundred years in the past, as far back as 700 years ago. An early example is the plus sign, which was used to mean "and" in a text made by a French mathematician.
Math was practiced as far back as Ancient Egypt, with numbers denoted with tally marks as opposed to the numerals that are used in modern times. Over the years, the symbols used in mathematical notation changed and varied wildly between countries and individuals, because there was no centralized system for a very long time.
Acceptance of the mathematical symbols that people are familiar with today is thanks to the creation of a system that began hundreds of years ago. The symbols that create the formulas and equations that are taught in schools in the United States stem from ideas from Ancient Greece and Germany, and Hindu-Arabic practices. Symbols as simple as the plus and minus signs were used as early as 1489 by a German mathematician, while the multiplication symbol is credited as being first created and used by an English man in the late 1500s. Another Englishman is responsible for the equals sign, having crafted and explained it in detail in 1557.