Posted by **Schuyler** on Monday, June 20, 2011 at 10:02am.

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

- Alg 2 -
**Raymond**, Monday, June 20, 2011 at 10:27am
Schuyler, the sum of the 12 term aritmetic progression is 816

- Alg 2 -
**Raymond**, Monday, June 20, 2011 at 11:17am
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

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