Posted by **Daniel** on Sunday, June 21, 2009 at 11:47pm.

How do you find the sum number of an arithmetic sequence? I forgot and my textbook does not explain it well.

- arithmetic -
**Reiny**, Sunday, June 21, 2009 at 11:57pm
depends what information you know

1. If you know the first term a, and the common difference d, and the number of terms n, then

Sum(n) = n/2[2a + (n-1)d]

2. If you know the first term a and the last term l, and the number of terms n, then

Sum(n) = n/2(a+l)

- arithmetic -
**Daniel**, Monday, June 22, 2009 at 12:04am
N = the term number right?

- arithmetic -
**PsyDAG**, Monday, June 22, 2009 at 11:23am
I'm not sure what you mean by "term number."

"n" typically refers to how many units you have, how many numbers you are dealing with, the number of scores.

"a" is the value of the first number. Is that what you mean by "term number"?

I hope this helps a little more. Thanks for asking.

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