have an arithmetic sequence here....to find the common difference, subtract the first term from the second term. 13 - 3 = 10...so basically, ur adding 10 to each number to get to the next number.
before we find s5, we r going to find a5...because we need a5 to use in the sum formula.....a5 is the fifth term, and it is 43
or, for this particular problem, s5 is the sum of the first 5 terms. 3 + 13 + 23 + 33 + 43 = 115.....u can do it this way for s5...but if u were asked to find s20, I suggest using the formula.
