What is the sum of the arithmetic sequence 22, 13, 4?

1 Answer
Sep 27, 2014

#22 + 13 + 4 = 39#. A very straightforward answer.

However, if one wishes to find it that way (the stupid way), then first notice that this is a simple arithmetic series. Use the formula #S = n/2 (a_1 + a_{\text{n}))# or the formula #S = n/2 (2a_1 + (n-1)d)# where #a_1# is the first term, #a_n# is the last term, #d# is the common difference, #n# is the number of terms in your series.

Note that both of these are equivalent since #a_n = a_1 + (n- 1)d#.

#S = 3/2 (22 + 4) = 39# or #S = 3/2 ( 2\cdot 22 - 9\cdot 2) = 39.#