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**Nikster****Member**- Registered: 2013-06-12
- Posts: 3

Find the first five terms of an arithmetic series with S10 = 210 and S20 = 820

S= Sum

*Last edited by Nikster (2013-06-12 14:54:47)*

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 86,676

Hi;

I am getting:

3, 7, 11, 15, 19, ...

**In mathematics, you don't understand things. You just get used to them.Of course that result can be rigorously obtained, but who cares?Combinatorics is Algebra and Algebra is Combinatorics.**

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**Nikster****Member**- Registered: 2013-06-12
- Posts: 3

I missed this class on how you do this. What process did you go through to get these numbers?

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 86,676

I solved 2 equations simultaneously: This is the formula for an artihmetic sum.

we have

this means we have 2 n's for equation1. With 2 n's we can get a1 ( the first term ) and d ( the distance between each term ). We have the 2 n's they are n =10 and n = 20.

Substituting into 1:

Clean em up:

Solving:

a1 = 3

d = 4

Can you finish up now?

**In mathematics, you don't understand things. You just get used to them.Of course that result can be rigorously obtained, but who cares?Combinatorics is Algebra and Algebra is Combinatorics.**

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**Nikster****Member**- Registered: 2013-06-12
- Posts: 3

I understand! Thank you so much!

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 86,676

Hi Nikster;

Your welcome and welcome to the forum!

**In mathematics, you don't understand things. You just get used to them.Of course that result can be rigorously obtained, but who cares?Combinatorics is Algebra and Algebra is Combinatorics.**

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