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
sn = (n (a1 + a5) / 2
s5 = (5 (3 + 43) / 2
s5 = (5 (46)) / 2
s5 = 230/2
s5 = 115 <===
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.
