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# MEI Core 2 Arithmetic Sequences? Help! watch

1. Right, question 7 for anyone that may for some unknown reason have the sheet, but for those that don't:

The 7th term of an arithmetic sequence is -2 and the 12th term is 8
i) Find the first term and the common difference.

(I got this as being common difference: 2, and first term as -14. If i'm wrong, please correct me here!)

ii) The sum of the terms is 364
Find the number of terms in the sequence.

This is when i'm seriously lost. I thought you needed to have the last term to figure out number of terms? Which I don't seem to have
2. First of all, good luck for tomorrow! I'm sitting it too. It's a retake, I thought I understood series and sequences last year, but now I really do.

Second off, your (i) is correct. For (ii), there are two formulae - one where you need the last term, another where you don't. Do you have your formula book in front of you? Quickly Google OCR MEI maths formula booklet, and go to page 2, it's the first one when you search for it I think. Do you remember the formula Sn = 1/2n(2a +[n-1]d)? That's the one you need. Put in the numbers you worked out from (i), make it equal to 364, solve the quadratic that pops out, and you get n = 28 or -14. Usually I wouldn't give you the answer, but since the exam is tomorrow this gives you a chance to work it out! If you still can't get it, I'll post up my working. Good luck!! x
3. Ahhh perfect! Thank you so much for your help!
Hope the exam went well x

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Updated: January 18, 2010
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