# Math

posted by .

Find the sum of the arithmetic series.
20
(Sigma Sign) 2n + 7
x= 1
Formula: Sn= a1 (1 - r^n / 1 - r)

• Math -

The formula you wrote down is applicable for geometric series. I would suggest that you study the derivations of the formulae for arithmetic and geometric series. It's no use compute the answers using formulae that you don't understand.

In case of this problem:

Summation of 2n + 7 =

2 [Summation of n] + 7 [summation of 1]

Summation of n from 1 to 20 =

1/2 Summation of n from 1 to 20 +

1/2 Summation of n from 1 to 20 =

1/2 Summation of n from 1 to 20 +

1/2 Summation of n from 20 to 1 =

1/2 Summation of n from 1 to 20 +

1/2 Summation of (21 - n) from 1 to 20 =

1/2 Summation of 21 from 1 to 20 =

1/2 21*20

So, the summation is 21*20 + 7*20 =28*20 = 560

## Similar Questions

1. ### Math

the sum of the first n terms of an arithmetic series is -51. the series has a constant difference of -1.5 and a first term of four. Find the number of terms in the sum I find using an EXCEL spreadsheet is very helpful for these types …
2. ### algebra

1)Find a1 in a geometric series for which Sn=300,r=-3,and n=4 a.15 b.15/2 c.-15 d.1/15 answer=a 2)Find the sum of the infinite geometric series n=1 at the bottom of the sigma notation 20(-1/4)n-1 on the right side a.25 b.80/3 c.16 …
3. ### algebra(check)

1)Find a1 in a geometric series for which Sn=300,r=-3,and n=4 a.15 b.15/2 c.-15 d.1/15 answer=a 2)Find the sum of the infinite geometric series n=1 at the bottom of the sigma notation 20(-1/4)n-1 on the right side a.25 b.80/3 c.16 …
4. ### algebra

1)Find a1 in a geometric series for which Sn=300,r=-3,and n=4 a.15 b.15/2 c.-15 d.1/15 answer=a 2)Find the sum of the infinite geometric series n=1 at the bottom of the sigma notation 20(-1/4)n-1 on the right side a.25 b.80/3 c.16 …
5. ### algebra

1)Find a1 in a geometric series for which Sn=300,r=-3,and n=4 a.15 b.15/2 c.-15 d.1/15 answer=a 2)Find the sum of the infinite geometric series n=1 at the bottom of the sigma notation 20(-1/4)n-1 on the right side a.25 b.80/3 c.16 …
6. ### math

Hi I need someone to check my answers I'll show whatever work I have but for time purposes I'll just post my answers because I think I have most of them correct. Thanks! 1)Find the 20th term of the arithmetic sequence in which a1=3 …
7. ### math

1)Find a1 in a geometric series for which Sn=300,r=-3,and n=4 A)15 B)15/2 C)-15 D)1/15 I chose A 2)Find the sum of the infinite geometric series. Sigma sign with infinity symbol above and n=1 below. To the right 20(-1/4)n-1 A)25 B)80/3 …
8. ### math(need 2nd opinion)

1)Find a1 in a geometric series for which Sn=300,r=-3,and n=4 A)15 B)15/2 C)-15 D)1/15 I chose A 2)Find the sum of the infinite geometric series. Sigma sign with infinity symbol above and n=1 below. To the right 20(-1/4)n-1 A)25 B)80/3 …
9. ### Algebra

find the indicated sum not sure how to type the sigma sign, but here is the problem 6 on top of the sigma sign. under the sigma sign is i=3 and on the right side of the sigma sign (3i-2)
10. ### Algebra 2

What is the sum of the arithmetic series 5 (sigma sign) n-1 (3n-2)?

More Similar Questions