Find S_n for the given arithmetic series.

a _1= 10, d = –9, n = 15

Question

Find S_n for the given arithmetic series.
 
a _1= 10, d = –9, n = 15

anonymous · Answer

And what does d represent?

rainbow_dash · Answer

common difference

callisto · Answer

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

rainbow_dash · Answer

is that first one a_1 or a_n

callisto · Answer

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

callisto · Answer

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

rainbow_dash · Answer

okay thanks :)

rainbow_dash · Answer

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

callisto · Answer

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

rainbow_dash · Answer

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?