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# Sigma notation: C2

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1. Hi, can some one please clarify the sigma notation for me please?
When there is, for example, n=3 on the bottom of the notation and 16 on the top, I just usually do 16-3=1=14 as my 'n' that I would sub into the equation... But I did a question today in which this wouldn't work. Meaning, I'd like to know the other method in which you make the n=1 at the bottom and take away another sum?
2. This is because you have misunderstood or at least missed one out. Imagine of the sum was from 1 to 2 Your technique would give one sum when in fact what we need is
3. Sorry, but i'm so confused. So how would you solve the notation, a=40, d=6
And the number on the bottom of the notation is 8
and the top is say, K, or 18?

I would just do 18-8+1=11
And use n=11?
4. (Original post by chanda01)
Hi, can some one please clarify the sigma notation for me please?
When there is, for example, n=3 on the bottom of the notation and 16 on the top, I just usually do 16-3=1=14 as my 'n' that I would sub into the equation... But I did a question today in which this wouldn't work. Meaning, I'd like to know the other method in which you make the n=1 at the bottom and take away another sum?
all the terms from 3 all the way to 16
huh? i don't understand post an example...
5. (Original post by chanda01)
Sorry, but i'm so confused. So how would you solve the notation, a=40, d=6
And the number on the bottom of the notation is 8
and the top is say, K, or 18?

I would just do 18-8+1=11
And use n=11?
Some sums do not work like that. eg a geometric series like a=10, r = 1.05. The sum from terms 1 to 10 is not the same as the sum from terms 11 to 20.

If you had something in that case and were asked to find the sum from 11 to 20, to use your formula you'd have to have things in terms of r = 1 to r = n, so you would find the sum of 1 to 20 and take away the sum of 1 to 10 (which leaves you with the sum of 11 to 20, intuitively).

It might work for something like an arithmetic series, where if you're looking for 11 to 20 for example you can set a= (whatever value it takes at r = 11) and use n = 20-11+1.

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