Question

find the sum of the first 20 terms in the arithmetic series -6-12-18

Answer 1

If a series is arithmetic the sum of the first n terms, denoted Sn, there are ways to find its sum without actually adding all of the terms.

To find the sum of the first n terms of an arithmetic series use the formula,
,
where n is the number of terms, a1 is the first term and an is the last term.

Example 1:

Find the sum of the first 20 terms of the arithmetic series if a1 = 5 and a20 = 62.



Example 2:

Find the sum of the first 40 terms of the arithmetic series 2 + 5 + 8 + 11 + ···.

First find the 40th term:



Then find the sum:



Example 3:

Find the sum:



First find a1 and a50:

a1 = 3(1) + 2 = 5

a50 = 3(50) + 2 = 152

Then find the sum:

Answer 2

did that help?

Answer 3

yea kind of

Answer 4

What else do you need?

Answer 5

Nice copy/paste @bleebleebleed

http://hotmath.com/hotmath_help/topics/sum-of-the-first-n-terms-of-an-arithmetic-series.html

Answer 6

I know i did

Answer 7

At least i took the time to do it @geerky42

Answer 8

Just so you can get medal, right...?

@cutegirl To find sum , you use this formula: \(S_n = \dfrac{n(a_n+a_1)}{2}\)

So all you need is first term (-6) and 20th term.

You have sequence -6,-12,-18,...

Can you find 20th term?

Answer 9

Nope I didnt want a medal