Monday

September 15, 2014

September 15, 2014

Posted by **Neil** on Tuesday, May 29, 2012 at 9:47pm.

20

(Sigma Sign) 2n + 7

x= 1

Formula: Sn= a1 (1 - r^n / 1 - r)

- Math -
**Count Iblis**, Tuesday, May 29, 2012 at 10:49pmThe 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

**Answer this Question**

**Related Questions**

Algebra - find the indicated sum not sure how to type the sigma sign, but here ...

math - Hi I need someone to check my answers I'll show whatever work I have but ...

math - 1)Find a1 in a geometric series for which Sn=300,r=-3,and n=4 A)15 B)15/2...

math(need 2nd opinion) - 1)Find a1 in a geometric series for which Sn=300,r=-3,...

algebra(check) - 1)Find a1 in a geometric series for which Sn=300,r=-3,and n=4 a...

algebra - 1)Find a1 in a geometric series for which Sn=300,r=-3,and n=4 a.15 b....

algebra - 1)Find a1 in a geometric series for which Sn=300,r=-3,and n=4 a.15 b....

algebra - 1)Find a1 in a geometric series for which Sn=300,r=-3,and n=4 a.15 b....

Math - the sum of the first n terms of an arithmetic series is -51. the series ...

Math(arithmetic & Geometric Series) - 1)The sum of 6 terms of an arithmetic ...