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March 22nd, 2014, 11:56 PM   #1
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Geometric progression.

Given a geometric progression 10,8,6.4,...., find the sum from the 3rd term to the 7th term of the geometric progression.
Why I can't get the answer 10? Can someone show me the solution?
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March 23rd, 2014, 02:39 AM   #2
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Re: Geometric progression.

You can use a formula ,

http://en.m.wikipedia.org/wiki/Geometric_progression

Using the formula in the link above for a geometric sum you would find the sum of the first 7 terms then subtract the sum of the first 2 terms.

I will do it the old fashioned (fun) way. The constant ratio is



Write down the 3rd to 7th terms



Now add them up







This does not reduce because there are no factors of 5 in niether 32 nor 2101 AND there are no factors of 2 in niether 5 nor 625. You can get a decimal approximation if you wish. Let us know if you have any questions about this or the wiki link referenced.

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March 23rd, 2014, 04:50 AM   #3
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Re: Geometric progression.

Quote:
Originally Posted by jiasyuen
Given a geometric progression 10,8,6.4,...., find the sum from the 3rd term to the 7th term of the geometric progression.
Why I can't get the answer 10? Can someone show me the solution?
As agentredlum pointed out .

That is "6.4" not "6,4" isn't it? With my bad vision, I thought that was an arithmetic progression! Yes, so it is the beginning of a geometric progression!
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March 23rd, 2014, 11:35 AM   #4
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Re: Geometric progression.

Quote:
Originally Posted by jiasyuen
Given a geometric progression 10,8,6.4,....,
Is that correct, or should it be 10,8,6,4? If so, it's arithmetic, not geometric...
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March 23rd, 2014, 02:24 PM   #5
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Re: Geometric progression.

In the Netherlands, among other countries, a comma (,) is used as a decimal mark. Though I've merely gotten used to use a point to, which sometimes leads to comments about it on Dutch boards.
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March 23rd, 2014, 05:23 PM   #6
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Re: Geometric progression.

Quote:
Originally Posted by jiasyuen
Given a geometric progression 10,8,6.4,...., find the sum from the 3rd term to the 7th term of the geometric progression.
Why I can't get the answer 10? Can someone show me the solution?
IF you're saying that 10 IS the given answer, then (as I pointed out earlier)
your 6.4 should be 6,4 (3rd and 4th terms) and the progression is ARITHMETIC;
6 + 4 + 2 + 0 - 2 = 10
Use google to find the formula...
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March 23rd, 2014, 05:33 PM   #7
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Re: Geometric progression.

Quote:
Originally Posted by Denis
Quote:
Originally Posted by jiasyuen
Given a geometric progression 10,8,6.4,...., find the sum from the 3rd term to the 7th term of the geometric progression.
Why I can't get the answer 10? Can someone show me the solution?
IF you're saying that 10 IS the given answer, then (as I pointed out earlier)
your 6.4 should be 6,4 (3rd and 4th terms) and the progression is ARITHMETIC;
6 + 4 + 2 + 0 - 2 = 10
Use google to find the formula...
No wonder I can't find the answer. I think my teacher has given me a wrong question. Thanks guys.
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March 23rd, 2014, 08:29 PM   #8
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Re: Geometric progression.

Well, if you're dealing with arithmetic series, and you want to calculate
the sum of the last m terms:
n = number of terms
a = 1st term
d = difference

Sum last m terms = n/2[2a + d(n-1)] - (m-1)/2[2a + d(m-2)]

Example: 2,5,8,11,14,17,20 : sum of last 4 ?

a = 2
d = 3
n = 7
m = 4

Sum last 4 = 7/2[2(2) + 3(7-1) - (4-1)/2[2(2) + 3(4-2)] = 62 ; 11+14+17+20 = 62

Got that?
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