You are not required to know explicit formulas for arithmetic or geometric series. In fact, I discourage my students from even learning "specialized" formula such as sum of 1, 2, ..., n, permutations, combination, binomial, etc. -- only basic formulae like N * average = sum, and D = R * T. Memorizing formulae distracted and discourages the understanding of the underlying principle.
However, you are expected to understand AVERAGES, and, with that understanding, you should be able to find the average and sum of ANY arithmetic sequence with ANY starting number.
Former Senior Instructor, Manhattan GMAT and VeritasPrep
Vice President, Midtown NYC Investment Bank, Structured Finance IT
MFE, Haas School of Business, UC Berkeley, Class of 2005
MBA, Anderson School of Management, UCLA, Class of 1993