Question

Find S_n for the given arithmetic series.

a _1= 10, d = –9, n = 15

Answer 1

And what does d represent?

Answer 2

common difference

Answer 3

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

Answer 4

is that first one a_1 or a_n

Answer 5

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

Answer 6

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

Answer 7

okay thanks :)

Answer 8

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

Answer 9

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

Answer 10

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?