Question

What is the sum of a 12-term arithmetic sequence where the last term is 13 and the common difference is -10

Answer 1

Start with getting the value of the first term. This you can get by using the formula \[a_n=a+(n-1)d\]

Answer 2

then what?

Answer 3

Given that there are 12 terms in the arithmetic sequence and the last term is 13, that is, 13 is the 12th term. So you can plug in n = 12, d = -10 (common difference), and \[a_{12}=13\] in the above formula to get the value of 'a'