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sigma notation - HSN forum

# sigma notation

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### #1Hev

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Posted 28 February 2007 - 08:10 PM

Find Sn= (n-1) above r=0 under 3^r

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### #2Steve

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Posted 01 March 2007 - 09:00 PM

OK, first of all just a general thing about the notation. It's nothing special! All means is:

When the number on top of the is finite (i.e. not ) this is called a partial (or finite) sum.

There are two special types of sequences: arithmetic and geometric. For these we have formulae for the nth partial sum.

You can check to see if the terms come from a geometric sequence by dividing any term by the term immediately before it. It is a geometric series if this number is the same no matter which term you do this for. This number is called the common ratio.

In our case, . So the common ratio is .

The formula for the sum to n terms where the terms come from a geometric sequence is:

where r is the common ratio and the first term of the sum is a. In our case .

Does this help?
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### #3Hev

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Posted 01 March 2007 - 11:52 PM

yeah i got it all now thanks

think i was just far too tired for maths.

know the feeling? haha

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### #4Steve

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Posted 04 March 2007 - 06:24 PM

Certainly do
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