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

    17
    ReputationRep:
    30
    ∑ (4r+7)
    r=3

    Hello, yes i know it's quite late.

    I'm very confused on how to solve this? i tried solving it the normal way i would if r=1, except by multiplying by 3 first. but the markscheme said it was wrong. (the answer should be 2044) how do i solve this? please explain.
    Offline

    21
    ReputationRep:
    Write out the first few terms to get the initial term and the difference term, then use the sum of an arithmetical series, which you can derive easily:

    S = a + (a+d) + .. (a+(n-1)d)
    S = (a+(n-1)d) + (a+(n-2)d) + .. a
    \therefore 2S = n[2a+(n-1)d]
    \therefore S = \frac{n}{2}[2a+(n-1)d]

    What values of a, d and n did you get?
    • Thread Starter
    Offline

    17
    ReputationRep:
    (Original post by RogerOxon)
    Write out the first few terms to get the initial term and the difference term, then use the sum of an arithmetical series, which you can derive easily:

    S = a + (a+d) + .. (a+(n-1)d)
    S = (a+(n-1)d) + (a+(n-2)d) + .. a
    \therefore 2S = n[2a+(n-1)d]
    \therefore S = \frac{n}{2}[2a+(n-1)d]

    What values of a, d and n did you get?
    a = 19
    d=4
    n=30

    is that correct??

    i tried using that very same formula before, but to no avail.
    Offline

    21
    ReputationRep:
    (Original post by ZiggyStarDust_)
    a = 19
    d=4
    n=30

    is that correct??
    Check your calculation of n.

    However, I don't get the answer that you gave
    • Thread Starter
    Offline

    17
    ReputationRep:
    (Original post by RogerOxon)
    Check your calculation of n.

    However, I don't get the answer that you gave
    im worried that my markscheme might be wrong?? 2044 is apparently the answer.

    So N is not 20? i don't understand what n could be then?
    • Community Assistant
    Offline

    18
    ReputationRep:
    (Original post by ZiggyStarDust_)
    30
    ∑ (4r+7)
    r=3

    Hello, yes i know it's quite late.

    I'm very confused on how to solve this? i tried solving it the normal way i would if r=1, except by multiplying by 3 first. but the markscheme said it was wrong. (the answer should be 2044) how do i solve this? please explain.
     \sum_{r=3}^{30}=\sum_{r=1}^{30} -\sum_{r=1}^{2}

    because you'd have  (u_1+u_2+u_3+u_4+\cdots +u_3) - (u_1 + u_2)

    can you see you'll be left with u_3 up to u_30 ?
    Offline

    21
    ReputationRep:
    r=3..30. If r were 1..30, you'd have 30 terms, so how many do you have for 3..30?
    Offline

    18
    ReputationRep:
    solve it like this https://gyazo.com/d401e7fef288e7176a9a7558275e8fdf
    Offline

    21
    ReputationRep:
    (Original post by ZiggyStarDust_)
    im worried that my markscheme might be wrong?? 2044 is apparently the answer.
    Sorry - I made a mistake - it is 2044.
    • Thread Starter
    Offline

    17
    ReputationRep:
    (Original post by rogeroxon)
    r=3..30. If r were 1..30, you'd have 30 terms, so how many do you have for 3..30?
    o shet its 27 isnt it
    Offline

    12
    ReputationRep:
    (Original post by ZiggyStarDust_)
    im worried that my markscheme might be wrong?? 2044 is apparently the answer.

    So N is not 20? i don't understand what n could be then?
    n=28 since the first term is 3 and the last term is 30 (so 28 terms all together).

    Now solve, the answer should be 2044.
    • Thread Starter
    Offline

    17
    ReputationRep:
    (Original post by tripplea)
    n=28 since the first term is 3 and the last term is 30 (so 28 terms all together).

    Now solve, the answer should be 2044.
    why would it be 28 as a pose to 27?

    because 30-3 is 27 right? so
    Offline

    12
    ReputationRep:
    (Original post by ZiggyStarDust_)
    why would it be 28 as a pose to 27?

    because 30-3 is 27 right? so
    If
    r=1-30 has 30 terms
    then
    r = 2-30 has 29 terms
    r = 3-30 has 28 terms
    • Thread Starter
    Offline

    17
    ReputationRep:
    (Original post by tripplea)
    If
    r=1-30 has 30 terms
    then
    r = 2-30 has 29 terms
    r = 3-30 has 28 terms
    thank you thank you thank you

    so much

    you've been very kind and helpful

    its very much appreciated
    you saved my arse

    thank you
    Offline

    16
    ReputationRep:
    30
    ∑ (4r+7)
    r=3

    19, 23, 27 - therefore the difference is 4
    a=19, d=4, n=28

    Sn = n/2[2a+(n-1)d]
    28/2[2(19)+(28-1)4)
    14[38+(27)4]
    Sn = 2044

    Hope that's clear enough for you
    Offline

    12
    ReputationRep:
    (Original post by ZiggyStarDust_)
    thank you thank you thank you

    so much

    you've been very kind and helpful

    its very much appreciated
    you saved my arse

    thank you
    You're welcome. Do you get it now?
 
 
 
  • 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?
    Help with your A-levels

    All the essentials

    The adventure begins mug

    Student life: what to expect

    What it's really like going to uni

    Rosette

    Essay expert

    Learn to write like a pro with our ultimate essay guide.

    Uni match

    Uni match

    Our tool will help you find the perfect course for you

    Study planner

    Create a study plan

    Get your head around what you need to do and when with the study planner tool.

    Study planner

    Resources by subject

    Everything from mind maps to class notes.

    Hands typing

    Degrees without fees

    Discover more about degree-level apprenticeships.

    A student doing homework

    Study tips from A* students

    Students who got top grades in their A-levels share their secrets

    Study help links and info

    Can you help? Study help unanswered threadsRules and posting guidelines

    Sponsored content:

    HEAR

    HEAR

    Find out how a Higher Education Achievement Report can help you prove your achievements.

    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.