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# core 2 geometric series help watch

1. Can someone please help me with this I've spent the last 2 hours trying to work it out seen as my notes don't seem to be helping in the slightest.

a) Find number of terms
(i) 2 + 4 + … + 1024

I know that im meant to use arn-1 but it doesn't seem to be working
2. Geometric series 2 + 4 + 8 + ... + 1024 = 2^1 + 2^2 + ... + 2^n
If I understand, you want to know what is n? Or do you want to find sum?
3. (Original post by baffled_mathman)
You can see that a=2 d=2. Knowing that Un=a+(n-1)d you can work out the number of terms (it's actually fairly obvious since every second number is a term of the series).

From this you can use the equation Sn = (n/2) a+l to find the answer. Where Sn is the sum to n terms and l is the last term.
The question is not clear I know BUT the titled suggests it is a GP and not an AP
4. (Original post by steve2005)
The question is not clear I know BUT the titled suggests it is a GP and not an AP
True, I read the title and deleted the post lol

For a GP it's similar though...use Un = ar^(n-1) to work out n, knowing that a and r = 2.

From that, use the sum equation.
5. (Original post by beccac94)
Can someone please help me with this I've spent the last 2 hours trying to work it out seen as my notes don't seem to be helping in the slightest.

a) Find number of terms
(i) 2 + 4 + … + 1024

I know that im meant to use arn-1 but it doesn't seem to be working
You do indeed want to use
You can see from the sequence what is and you can use

Now you can simply say and so we can now do:

and then you can also take logs of each side with base r to get

You can then use that to find n

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