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Math: Find the sum. Help?

0 votes
1) 1 + 3 +9 +....+2187

2) 1 - 1/2 + 1/ 4 - 1/8 +....- 1/512
asked May 27, 2013 in ALGEBRA 2 by skylar Apprentice

3 Answers

0 votes

1)

 The series is1 + 3 + 32 +...............37

a = 1, r  =3 and arn-1 = 37

33n-1 = 37

3n = 37

n = 7

The sum of the series  = a(r) / r - 1 = 3(3n-1) / 3 - 1

                                                                         = 3n / 2.

 

answered May 30, 2013 by diane Scholar
0 votes

Geometric sum is image

1) The geometric sum is 1 + 3 + 9 + .......+ 2187

image

Since image

image

n th term is

image

image

Since bases are equal,equate the powers.

image

image

Therefore, image

image

image

answered Jul 26, 2014 by david Expert
0 votes

2) The geometric sum is

 

image

Since

Formula for sum of the sequence

Where a = first term, r = common ratio.

Formula for n th term

Since bases are equal,equate the powers.

Therefore,image

image

image

image

answered Jul 26, 2014 by david Expert
edited Jul 26, 2014 by david

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