Question

What is the sum of a 23–term arithmetic sequence where the first term is 9 and the last term is 185?

Answer 1

sn=[n(a1+an) / 2]

a1=9
a23=185
n=23

Plug into the sn equation

Answer 2

Thanks ! Exactly what i needed.

Answer 3

@DeeJay23 do you know what formula a_n has?

Answer 4

uhmmm , Arthemetic Series ,i think ? im confused noww :/

Answer 5

\[\LARGE a_n=a_1+(n-1)d\]

but you don't have difference yet.. do you know how to find it?

Answer 6

just substitute

\[\LARGE a_{23}=a_1+(23-1)d\]
\[\LARGE 185=9+22d\]

solve for "d" then you have it. Once you're done with it... plug it back in the equation optiquest wrote:

\[\LARGE S_n=\frac n2[2a_1+(n-1)d]\]