## anonymous 3 years ago Find S_n for the given arithmetic series. a _1= 10, d = –9, n = 15

1. anonymous

And what does d represent?

2. anonymous

common difference

3. Callisto

Formula to use: $S_{n}=a_n + (n-1)d$

4. anonymous

is that first one a_1 or a_n

5. Callisto

I must be crazy somehow. Just a minute. Forget the previous post first!!

6. Callisto

Here it goes. $S_{n} = \frac{n}{2} [2a + (n-1)d]$ a = first term, d=common difference , n=number of terms

7. anonymous

okay thanks :)

8. anonymous

do'nt ask why but the question changed, and now i just need to write an equation.

9. Callisto

Sorry for my silly mistakes :( I suppose the first one I wrote was for $$a_n$$. but still, it's wrong! $$a_n=a_1+(n-1)d$$

10. anonymous

thank you for your help :) this newest one is to find a_n, and once i have a_n i use the 2nd one you put?