Hey there! Sign in to join this conversationNew here? Join for free
x Turn on thread page Beta

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

    • Thread Starter
    Offline

    2
    ReputationRep:
    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
    • Very Important Poster
    Online

    21
    ReputationRep:
    Very Important Poster
    (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?
    • Thread Starter
    Offline

    2
    ReputationRep:
    (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
     
    • Very Important Poster
    Online

    21
    ReputationRep:
    Very Important Poster
    (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.
    • Thread Starter
    Offline

    2
    ReputationRep:
    (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!
    • Very Important Poster
    Online

    21
    ReputationRep:
    Very Important Poster
    (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\sum_{r=0}^4 r^2 = 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
    \sum_{r=0}^4 r^2 is the same as \sum_{r=1}^4 r^2 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
    \sum_{r=1}^n r^2 = \frac{1}{6}n(n+1)(2n+1))
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: May 15, 2016
Poll
Do you agree with the proposed ban on plastic straws and cotton buds?
Useful resources

Make your revision easier

Maths

Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

Equations

How to use LaTex

Writing equations the easy way

Student revising

Study habits of A* students

Top tips from students who have already aced their exams

Study Planner

Create your own Study Planner

Never miss a deadline again

Polling station sign

Thinking about a maths degree?

Chat with other maths applicants

Can you help? Study help unanswered threads

Groups associated with this forum:

View associated groups

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

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