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# Need help on c2 question arithmetic watch

1. I only need help on C
I did the first part so I attached that as well

I don't understand that question at all, so If you could explain, how to do it then it would be very helpful, thank you

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2. (Original post by am99)

I only need help on C
I did the first part so I attached that as well

I don't understand that question at all, so If you could explain, how to do it then it would be very helpful, thank you

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The sum of the first 25 terms is given to you in the question so you can substitute that into the equation. If you then substitute x as the sum of the first k numbers, you get the equation 33(3500-x)=67x which you can then solve. Hope that helps
3. (Original post by LilacFlorence)
The sum of the first 25 terms is given to you in the question so you can substitute that into the equation. If you then substitute x as the sum of the first k numbers, you get the equation 33(3500-x)=67x which you can then solve. Hope that helps
Ok I understand thank you for your help !

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4. (Original post by LilacFlorence)
The sum of the first 25 terms is given to you in the question so you can substitute that into the equation. If you then substitute x as the sum of the first k numbers, you get the equation 33(3500-x)=67x which you can then solve. Hope that helps

Could you help me on that question,
You don't have to do it just show me so I can start it, both part of the question

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5. (Original post by am99)

Could you help me on that question,
You don't have to do it just show me so I can start it, both part of the question

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Question 6bi and ii or 7a and b?
6. (Original post by LilacFlorence)
Question 6bi and ii or 7a and b?
Question 6bi and ii

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7. (Original post by am99)
Question 6bi and ii

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Ok so for the first part you need to put the numbers you've been given into the equation Un = a + (n – 1)d.
Then for part ii) you know that Uk = 0 so you can replace Un with 0, and n with k in the equation you found in part i) so that you can find what k equals. Then you can use this value in the Sn = 1/2 n(a + l) equation.
Let me know if you'd like some more hints
8. (Original post by LilacFlorence)
Ok so for the first part you need to put the numbers you've been given into the equation Un = a + (n – 1)d.
Then for part ii) you know that Uk = 0 so you can replace Un with 0, and n with k in the equation you found in part i) so that you can find what k equals. Then you can use this value in the Sn = 1/2 n(a + l) equation.
Let me know if you'd like some more hints
Okay thanks, I will reply soon after I do

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9. (Original post by LilacFlorence)
Ok so for the first part you need to put the numbers you've been given into the equation Un = a + (n – 1)d.
Then for part ii) you know that Uk = 0 so you can replace Un with 0, and n with k in the equation you found in part i) so that you can find what k equals. Then you can use this value in the Sn = 1/2 n(a + l) equation.
Let me know if you'd like some more hints

Is this right so far? Afterward I am confused as to what to put in the equation, I use this one instead
1/2n(2a+(n-1)d
So 1/2n(2(240)+(n-1)d
But what is N, is it the k value or n=1?

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10. (Original post by am99)

Is this right so far? Afterward I am confused as to what to put in the equation, I use this one instead
1/2n(2a+(n-1)d
So 1/2n(2(240)+(n-1)d
But what is N, is it the k value or n=1?

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Yes that is right n is the k value
11. (Original post by LilacFlorence)
Yes that is right n is the k value
okay, i just checked and i got the answer right, thank you for your help sorry if i bothered you
12. (Original post by am99)
okay, i just checked and i got the answer right, thank you for your help sorry if i bothered you
No problem at all - let me know if there's anything else I can help with
13. (Original post by LilacFlorence)
The sum of the first 25 terms is given to you in the question so you can substitute that into the equation. If you then substitute x as the sum of the first k numbers, you get the equation 33(3500-x)=67x which you can then solve. Hope that helps
Sorry to bother you but how do you do part a?
14. (Original post by SANTR)
Sorry to bother you but how do you do part a?
You need to use the formula Un = a+(n-1)d and sub in the values you have been given
15. (Original post by LilacFlorence)
You need to use the formula Un = a+(n-1)d and sub in the values you have been given
Oh right, I was confused because the title states 'C2' and arithmetic series isn't a C2 topic for Edexcel.
16. (Original post by SANTR)
Oh right, I was confused because the title states 'C2' and arithmetic series isn't a C2 topic for Edexcel.
yeah that confused me too -I think it's C2 for AQA
17. (Original post by LilacFlorence)
No problem at all - let me know if there's anything else I can help with

Could you help me on part a, I know it's a simultaneous equation but I just can't set up the equation, i find it so confusing

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18. (Original post by am99)

Could you help me on part a, I know it's a simultaneous equation but I just can't set up the equation, i find it so confusing

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So using the equation you were given, you can form the equation 132 = 160p +q.
Then if you know the limit is 20 as it tends to infinity you can sub 20 into the equation L = pL + q to get 20 =20p + q and then solve the two equations simultaneously.
On the exam board I do we don't learn about limits - I just looked up about them quickly but sorry I can't really explain the limits bit any more than that!
19. (Original post by LilacFlorence)
So using the equation you were given, you can form the equation 132 = 160p +q.
Then if you know the limit is 20 as it tends to infinity you can sub 20 into the equation L = pL + q to get 20 =20p + q and then solve the two equations simultaneously.
On the exam board I do we don't learn about limits - I just looked up about them quickly but sorry I can't really explain the limits bit any more than that!
Ok thank you, sorry for the late Reply, I forgot about it completely
Could you help me on part C please.

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20. (Original post by am99)
Ok thank you, sorry for the late Reply, I forgot about it completely
Could you help me on part C please.

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You need to use the equation Sn =1/2n[2a+(n−1)d]
You know from part a and b that a=87 and d=-3 and you are told that n=k and Sn=0 so sub these numbers in and solve to find k

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