Hey there! Sign in to join this conversationNew here? Join for free
    • Thread Starter
    Offline

    2
    ReputationRep:
    Having a bit of trouble with these;

    1) Find in a simplified form \displaystyle\sum_{r=1}^n (7r-2)
    What is this actually asking for?

    2) Someone invests £20 one month, then 25, then 30, then 35 and so on until he has made an investment £500. How much has she invested in total?

    Is this using
    Number of terms x sum of first and last / 2
    So (96 x 520)/2 = £24 960

    3) Find the first 4 terms in the sequences whos nth term is given by
    Un = \frac{n}{n+1}

    4) Find the first 4 terms of sequence defined as follows;
    t_{n+1} = 3t_n -1

t_1 = 4

    5)3 consecutive terms of an arithmetic sequence are

    3p-2, 4p-6, 9p+16

    Find the value of p and the common difference.

    -----------

    +rep to help, even if its just the formula to use etc.
    Offline

    0
    ReputationRep:
    (Original post by jbeach09)
    Having a bit of trouble with these;

    1) Find in a simplified form \displaystyle\sum_{r=1}^n (7r-2)
    What is this actually asking for?
    I'm only going to look at the first one.

    It says what it wants but to help...

    \displaystyle\sum_{r=1}^n (7r-2) =7\displaystyle\sum_{r=1}^n r-\displaystyle\sum_{r=1}^n 2 and you should know what \displaystyle\sum_{r=1}^n r is.
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by Get me off the £\?%!^@ computer)
    I'm only going to look at the first one.

    It says what it wants but to help...

    \displaystyle\sum_{r=1}^n (7r-2) =7\displaystyle\sum_{r=1}^n r-\displaystyle\sum_{r=1}^n 2 and you should know what \displaystyle\sum_{r=1}^n r is.
    I got that far with the summation results page in my book, but I've got no idea what the next step is.

    I was absent through most of the series lessons due to illness and have had this assignment, the questions with find the nth term are ok because I was in that, but these I've got no idea.

    Surely that's saying the last value of r in the sequence is n, and the first value is 1, but whats the method next?
    Offline

    0
    ReputationRep:
    (Original post by jbeach09)
    I got that far with the summation results page in my book, but I've got no idea what the next step is.

    I was absent through most of the series lessons due to illness and have had this assignment, the questions with find the nth term are ok because I was in that, but these I've got no idea.

    Surely that's saying the last value of r in the sequence is n, and the first value is 1, but whats the method next?
    \displaystyle\sum_{r=1}^n r=\frac{n}{2}(n+1)

    Post some working if you need any more help.
    Offline

    0
    ReputationRep:
    i also hate this topic but for the second one,
    you use the formula : Sn: n/2(a +l)
    so ur first term is 20 and your last term is 500, sub in those and work your way to the anser

    for the 3rd one, just sub in 1 for n and do the same for 2 and 3 and 4 as they want the first 4

    for the 4th one, ur meant to be using the recurrance method, so sub in 1 for n and they tell u that t1=4 so 4 times 3 and then minus one. then u will get t2 and use t2 to find t3 and so forth

    the fifth and the first one are tricky
    good luck
    • Thread Starter
    Offline

    2
    ReputationRep:
    Thanks, managed to do them all apart from the last one and first one now

    (Original post by Get me off the £\?%!^@ computer)
    \displaystyle\sum_{r=1}^n r=\frac{n}{2}(n+1)

    Post some working if you need any more help.
    So would you do

    7(\frac{n}{2}(n+1))-\frac{n}{2}(n+1)2
    or am I barking up the wrong tree?

    If I still cannot get it, it's probably worth me going and having a word with my teacher tomorrow.
    Offline

    0
    ReputationRep:
    (Original post by jbeach09)
    Thanks, managed to do them all apart from the last one and first one now


    So would you do

    7(\frac{n}{2}(n+1))-\frac{n}{2}(n+1)2
    or am I barking up the wrong tree?

    If I still cannot get it, it's probably worth me going and having a word with my teacher tomorrow.
    This bit

    \displaystyle\sum_{r=1}^n 2

    is just nx2.

    Yes talking to your teacher is a good idea.
 
 
 
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • Poll
    What newspaper do you read/prefer?
    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
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • 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

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