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 March 22nd, 2014, 11:56 PM #1 Senior Member   Joined: Sep 2013 From: Earth Posts: 827 Thanks: 36 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? March 23rd, 2014, 02:39 AM #2 Math Team   Joined: Jul 2011 From: North America, 42nd parallel Posts: 3,372 Thanks: 233 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.  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! 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... March 23rd, 2014, 02:24 PM #5 Math Team   Joined: Apr 2010 Posts: 2,780 Thanks: 361 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. 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... 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. March 23rd, 2014, 08:29 PM #8 Math Team   Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 14,597 Thanks: 1038 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? Tags geometric, progression ,

### Geometric progression 1st is 7 and 4th is 56 then 7th term is

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