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# FP1 OCR June 2014 Q8 Help watch

1. I'm doing the OCR June 2014 FP1 paper, but I'm not sure how to go about doing question 8. I know I'm meant to take off the sum of n-1 from 2n from the mark scheme, but I'm not sure why that would work - I know I have to substitute in 2n as n, but I wasn't sure how to get it to start from 1 at the bottom (I don't see how taking away the summation with n-1 as n would work)> Could someone explain how this works?

Question paper:http://www.ocr.org.uk/Images/242444-...hematics-1.pdf

Mark scheme: http://www.ocr.org.uk/Images/235974-...ics-1-june.pdf
2. (Original post by SpaghettiGuy1337)
I'm doing the OCR June 2014 FP1 paper, but I'm not sure how to go about doing question 8. I know I'm meant to take off the sum of n-1 from 2n from the mark scheme, but I'm not sure why that would work - I know I have to substitute in 2n as n, but I wasn't sure how to get it to start from 1 at the bottom (I don't see how taking away the summation with n-1 as n would work)Question paper:http://www.ocr.org.uk/Images/242444-...hematics-1.pdfMark scheme: http://www.ocr.org.uk/Images/235974-...ics-1-june.pdf
It should work, I'll have a go if you want
3. (Original post by SpaghettiGuy1337)
I'm doing the OCR June 2014 FP1 paper, but I'm not sure how to go about doing question 8. I know I'm meant to take off the sum of n-1 from 2n from the mark scheme, but I'm not sure why that would work - I know I have to substitute in 2n as n, but I wasn't sure how to get it to start from 1 at the bottom (I don't see how taking away the summation with n-1 as n would work)Question paper:http://www.ocr.org.uk/Images/242444-...hematics-1.pdfMark scheme: http://www.ocr.org.uk/Images/235974-...ics-1-june.pdf
Think about the definition of sigma.

For example, let's say I wanted to sum .

We have:

And:

Subtracting those two:

So, as you can see, if you want to sum between two number , you can instead sum from 1 to 2n and to get and then subtract away the first 1 to (n-1) terms to get: as required.

Hence:
4. (Original post by Zacken)
Think about the definition of sigma.

For example, let's say I wanted to sum .

We have:

And:

Subtracting those two:

So, as you can see, if you want to sum between two number , you can instead sum from 1 to 2n and to get and then subtract away the first 1 to (n-1) terms to get: as required.

Hence:
That's really well explained. Thank you so much.
5. (Original post by SpaghettiGuy1337)
That's really well explained. Thank you so much.
Glad it helped, thank you! Appreciated.

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