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# arithmetic sequences watch

Attachment 730620
2. (Original post by HiggsBoson)
Attachment 730620

3. I think this might work.
4. (Original post by HiggsBoson)

I think this might work.
Which part do you need help with? Also, please post what you've tried.
5. I have done part a and got:
A2=5-4k,
A3=5-5k-4k^2,
I know these are right as my teacher has marked them correct.
Im not sure how to work out the answers using sigma notation though.
6. (Original post by HiggsBoson)
I have done part a and got:
A2=5-4k,
A3=5-5k-4k^2,
I know these are right as my teacher has marked them correct.
Im not sure how to work out the answers using sigma notation though.
Do you mean you can't do b) and c)?
7. (Original post by Notnek)
Do you mean you can't do b) and c)?
Yeah
8. (Original post by HiggsBoson)
Yeah
So for b) if you plug r = 1 into the exression inside the sigma you get

then plug r = 2

and r = 3

So

9. (Original post by Notnek)
So for b) if you plug r = 1 into the exression inside the sigma you get

then plug r = 2

and r = 3

So

I got 17-9K-4K^2, is this correct?
10. (Original post by HiggsBoson)
I got 17-9K-4K^2, is this correct?
The numbers are correct but the signs are wrong. Please post your working.
11. (Original post by Notnek)
The numbers are correct but the signs are wrong. Please post your working.
(1+4)+(1+5-4k)+(1+5-5K-4k^2)
I added up all the like terms.
12. (Original post by HiggsBoson)
I have done part a and got:
A2=5-4k,
A3=5-5k-4k^2,
I know these are right as my teacher has marked them correct.
Im not sure how to work out the answers using sigma notation though.
I just noticed that your A3 is incorrect.

It should be which becomes

13. (Original post by Notnek)
I just noticed that your A3 is incorrect.

It should be which becomes

So the correct answer should be 17-9k+4k^2?
14. (Original post by HiggsBoson)
So the correct answer should be 17-9k+4k^2?
Yes that's right.
15. (Original post by Notnek)
Yes that's right.
Thank you,
16. (Original post by HiggsBoson)
Thank you,
Try plugging in r = 1 then r = 2 etc. like I showed you in part b). Do this a few times and you should notice a pattern. Post your working if you're unsure.
17. (Original post by Notnek)
Try plugging in r = 1 then r = 2 etc. like I showed you in part b). Do this a few times and you should notice a pattern. Post your working if you're unsure.
Sorry I don't understand, isn't it the sum of the first 100 terms.
18. (Original post by HiggsBoson)
Sorry I don't understand, isn't it the sum of the first 100 terms.
Yes it is but there's a trick to doing this - you don't have to find all 100 terms. Try finding the first few and see what you get. Post your working if you get stuck.
19. (Original post by Notnek)
Yes it is but there's a trick to doing this - you don't have to find all 100 terms. Try finding the first few and see what you get. Post your working if you get stuck.
I'm starting to notice a pattern:
a1=4
a2=5-4k
a3=5-5k+4k^2
a4=5-5k-5k^2-4k^3
a5=5-5k-5k^2-5k^3-4k^4.
Im not sure what to do next though
20. (Original post by HiggsBoson)
I'm starting to notice a pattern:
a1=4
a2=5-4k
a3=5-5k+4k^2
a4=5-5k-5k^2-4k^3
a5=5-5k-5k^2-5k^3-4k^4.
Im not sure what to do next though
For c) you need to consider the expression inside the sigma so for r = 1:

Try doing it for r = 2 and r = 3.

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