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
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C1 - Where am I going wrong with the sum of an arithmetic series? watch
- Thread Starter
- 12-01-2017 16:33
- Study Helper
- 12-01-2017 17:12
- Thread Starter
(Original post by ghostwalker)
- 15-01-2017 16:51
You only went wrong on your last line.
But then you've multiplied by the 2, rather than dividing.
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?
ThanksLast edited by blobbybill; 15-01-2017 at 17:09.
- 15-01-2017 18:02
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.
- 15-01-2017 18:07
If you multiply by two you end up with:
2Sn = 15(56)
You have actually multiplied by four, which gives:
4Sn = 15(112)