I do not have an formal math education, but I'm fairly sure what you quote is part of the Arithmetic Progression concept (the sum formula more specifically), so taking a look at the arithmetic branch of math might be a good idea.

Wikipedia[0], MathWords[1] and Brilliant[2] might be some good resources for you.

[0] https://en.wikipedia.org/wiki/Lists_of_mathematics_topics [1] http://www.mathwords.com/ [2] https://brilliant.org/