Hey! Sign in to get help with your study questionsNew here? Join for free to post
 You are Here: Home >< Maths

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

Announcements Posted on
TSR's new app is coming! Sign up here to try it first >> 17-10-2016
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
2. (Original post by cookiemunch12)
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.
Attached Images

4. (Original post by cookiemunch12)
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!
6. (Original post by cookiemunch12)
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

Write a reply…

Submit reply

## Register

Thanks for posting! You just need to create an account in order to submit the post
1. this can't be left blank
that username has been taken, please choose another Forgotten your password?
2. this can't be left blank
this email is already registered. Forgotten your password?
3. this can't be left blank

6 characters or longer with both numbers and letters is safer

4. this can't be left empty
your full birthday is required
1. Oops, you need to agree to our Ts&Cs to register
2. Slide to join now Processing…

Updated: May 15, 2016
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Today on TSR

### How does exam reform affect you?

From GCSE to A level, it's all changing

Poll
Useful resources

## Make your revision easier

### Maths Forum posting guidelines

Not sure where to post? Read here first

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams

Can you help? Study help unanswered threads

## Groups associated with this forum:

View associated groups
Study resources

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.