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

use the formula n/2(2a+(n-1)d)

Answer 2

the formula for sum of arithmetic sequence \[\large S_n = \frac{n(A_1 + A_n)}{2}\]

Answer 3

however the first term was not given so you solve for it using \[\large A_n = A_1 + (n-1)d\]

Answer 4

do you need any more help @cw7652