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# C1 - Where am I going wrong with the sum of an arithmetic series? watch

1. I got part A correct with the values for a and d being correct too.

However, when it comes to part B of the question, I get it wrong. The answer is 420 according to the mark scheme. I am using the equation: Sn = N/2 [2a+(n-1)d]

The question is this: An arithmetic series has first term 49 and 15th term 7. Part A) Find the value of the common difference (I got this correct)
Part B) Find the value of the sum of the first 15 terms of the series

How am I going wrong? I don't understand. Thank you

2. (Original post by blobbybill)
...
You only went wrong on your last line.

That's fine.

But then you've multiplied by the 2, rather than dividing.
3. (Original post by ghostwalker)
You only went wrong on your last line.

That's fine.

But then you've multiplied by the 2, rather than dividing.
Why do you divide the 56 by 2 in order to make it 15*28? I don't get how you know that is possible. Why can't you just multiply it all by 2 in order to eliminate the denominator like I did here?

And how can I tell whether I can multiply by the denominator to eliminate the fraction and then solve from there, or whether I have to do it like you just said? In AS level maths so far, I swear I have been able to solve things by multiplying everything by a denominator in order to get rid of a fraction and then continue from there (I defo remember doing it in coordinate geometry). Why can't I do that here? How would I be able to tell whether or not I can do that?

Thanks
4. If you multiply the right hand side by 2, you have to do the same to the left hand side to keep the equation balanced.

Another way of looking at this is that (15/2)=7.5. It therefore follows that (15/2)x56=7.5x56=420.
5. If you multiply by two you end up with:

2Sn = 15(56)

You have actually multiplied by four, which gives:

4Sn = 15(112)

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