Here's the question you clicked on:
Rainbow_Dash
Find S_n for the given arithmetic series. a _1= 10, d = –9, n = 15
And what does d represent?
Formula to use: \[S_{n}=a_n + (n-1)d\]
is that first one a_1 or a_n
I must be crazy somehow. Just a minute. Forget the previous post first!!
Here it goes. \[S_{n} = \frac{n}{2} [2a + (n-1)d]\] a = first term, d=common difference , n=number of terms
do'nt ask why but the question changed, and now i just need to write an equation.
Sorry for my silly mistakes :( I suppose the first one I wrote was for \(a_n\). but still, it's wrong! \(a_n=a_1+(n-1)d\)
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?