Question

Will medal...
Find the sum of the following series.

Answer 1

A. 240
B. 255
C. 210
D. 510

Answer 2

It's n = 1 not n - 1. Sorry.

Answer 3

\[\huge \sum_{n=1}^{15}(2n+1)\]

Answer 4

Yes, just like that.

Answer 5

Yes so you start at n=1 and end at n=15

Answer 6

We have 15 terms actually.

Answer 7

Okay, how do I plug it in?

Answer 8

Getting ahead of myself - first we need to find n=1 and n=15

Answer 9

To find this you will plug in 1 and 15 into 2n+1

Answer 10

what did you get when you plug in 1 and then 15?

Answer 11

I'm not sure how to plug it in. I've never done these problems before.

Answer 12

when n=1 and n=15 plug it in. 2n+1 meaning plug 1 and 15 for n

Answer 13

So 2(1)+15?

Answer 14

No.

Answer 15

Plug in 1 and 15 for n. You will have 2 answers.

Answer 16

2n+1 when n is 1 and 15

Answer 17

Like this?
2(1) + 1 = 3
and
2(15) + 1 = 31

Answer 18

There we go.

So a_1=3 and a_n=31

a_1 is the first terms and a_n represents the last term which in this case is a_15

n=15 because there is a total of 15 terms

Answer 19

Plug all that into the top/first formula

Answer 20

So 15/2 (3 + 31) ??

Answer 21

That would give me 255.

Answer 22

Great job! 255 is the sum of this arithmetic series.