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

Schuyler, the sum of the 12 term aritmetic progression is 816

Answer 2

Schuyler, in order to solve your problem I had to turn it around (Gloria Estefan?). You said the last term was 13...I made it the first term. You said the common difference was a -10...I made it a +10.
Now I had to find the last term so I used this formula: L=a+(n-1)d so 13+(12-1)10
L=123
Then I used this formula:
Sn=n/2(a+L)
Sn=12/2(13+123)
Sn=6 Times 136
Sn=816