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# C1 Arithmetic Sequences - When do you use n and n-1 watch

1. The formula is a(n-1)d but ive noticed that for some questions normally when you're dealing with years, you only use a*n*d and not a(n-1)d. Can someone clarify when this is the case and tell my why?

Edit: Arithmetic series not sequences
The formula is a(n-1)d but ive noticed that for some questions normally when you're dealing with years, you only use a*n*d and not a(n-1)d. Can someone clarify when this is the case and tell my why?

Edit: Arithmetic series not sequences
Could you show us an example of a question using a*n*d, please?
3. (Original post by SeanFM)
Could you show us an example of a question using a*n*d, please?
Attached. Oh I just realized I use *. I meant a + nd. But either way I still think we use a +(n-1)d but 2000 to 2007 counts as 8 years.
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Attached. Oh I just realized I use *. I meant a + nd. But either way I still think we use a +(n-1)d but 2000 to 2007 counts as 8 years.
Ah, I see. That is more interpreting what n is, which is +1 more than you might think, which is why it looks like it's a +nd instead of a + (n-1)d.

If you count using your fingers for each year, then 2000 is the first year, 2001 is the second year... and so 2007 is the 8th year, so you are using a + (n-1)d, just with n = 8 when the gut instinct would be to go with n = 7.
5. (Original post by SeanFM)
Ah, I see. That is more interpreting what n is, which is +1 more than you might think, which is why it looks like it's a +nd instead of a + (n-1)d.

If you count using your fingers for each year, then 2000 is the first year, 2001 is the second year... and so 2007 is the 8th year, so you are using a + (n-1)d, just with n = 8 when the gut instinct would be to go with n = 7.
Yes I understand thanks. So if we had ∑ with 15 at the top and 11 at the bottom n would be 5 and ∑ with 15 at the top and 10 at the bottom it would be 6.

Its pretty obvious. Dont know what I was thinking but thank you for your help!
Yes I understand thanks. So if we had ∑ with 15 at the top and 11 at the bottom n would be 5 and ∑ with 15 at the top and 10 at the bottom it would be 6.

Its pretty obvious. Dont know what I was thinking but thank you for your help!
Yes, that's correct .It's because the first r term or i term or whatever your index is is put in, then you put in the next value, and the next value, and the last value you put in is the value on top, and you add all the terms together.

Eg = 0^2 + 1^2 + 2^2 +3^2 + 4^2 = 30. (and as a side note, if you want to do this using the formula for sum of r^2 then just realise that
is the same as because 0^2 = 0 so it adds nothing to the series, and 0^2 is the difference between the two, then you put in n= 4 into

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