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

    7
    ReputationRep:
    Evaluate

    30 r=10∑ (7 + 2r)

    30 is above the sigma sign and r=10 is below
    • Community Assistant
    Offline

    18
    ReputationRep:
    Community Assistant
    split it so it's like this;

    \sum_{10}^{30} (7+2r) =\sum_{1}^{30} (7+2r) -\sum_{1}^{9} (7+2r)
    • Thread Starter
    Offline

    7
    ReputationRep:
    (Original post by NotNotBatman)
    split it so it's like this;

    \sum_{10}^{30} (7+2r) =\sum_{1}^{30} (7+2r) -\sum_{1}^{9} (7+2r)
    can you explain it further and what you'd do?
    the mark scheme uses n/2 ( a + L) i believe which isnt what we've learnt
    Offline

    2
    ReputationRep:
    since when is this c1??, what board are you doing ?
    • Community Assistant
    Offline

    18
    ReputationRep:
    Community Assistant
    (Original post by Mina_)
    can you explain it further and what you'd do?
    the mark scheme uses n/2 ( a + L) i believe which isnt what we've learnt
    S_n =\frac{n}{2} (a+l) where a is the first term and l is the last term, it's a standard formula which you should learn.

    So if you have to sum from  r=10 to  r=30 then that's

     U_1_0 + U_11 + U_1_2 + ... + U_3_0,

    but this formula uses the first term, so you have to realise that the sum from  r=10 to  r = 30 is the same as summing from  r=1 to r=30 subtract the sum form  r =1 to r=9

    because  \sum_{1}^{30}= u_1 + u_2 + u_3 +u_4 + u_5 + u_6 + u_7 + u_8 + u_9 + u_1_0+ u_1_1+...+u_3_0

    and \sum_{1}^{9} = u_1 + u_2 + u_3 +u_4 + u_5 + u_6 + u_7 + u_8 + u_9

    and notice when you subtract them you're left with the summation form  u_1_0 to  u_3_0
    which is what you're asked for, so apply the summation formula remembering a is the first term (the number at the bottom) and l is the last term (the number at the top).
    • Thread Starter
    Offline

    7
    ReputationRep:
    (Original post by NotNotBatman)
    S_n =\frac{n}{2} (a+l) where a is the first term and l is the last term, it's a standard formula which you should learn.

    So if you have to sum from  r=10 to  r=30 then that's

     U_1_0 + U_11 + U_1_2 + ... + U_3_0,

    but this formula uses the first term, so you have to realise that the sum from  r=10 to  r = 30 is the same as summing from  r=1 to r=30 subtract the sum form  r =1 to r=9

    because  \sum_{1}^{30}= u_1 + u_2 + u_3 +u_4 + u_5 + u_6 + u_7 + u_8 + u_9 + u_1_0+ u_1_1+...+u_3_0

    and \sum_{1}^{9} = u_1 + u_2 + u_3 +u_4 + u_5 + u_6 + u_7 + u_8 + u_9

    and notice when you subtract them you're left with the summation form  u_1_0 to  u_3_0
    which is what you're asked for, so apply the summation formula remembering a is the first term (the number at the bottom) and l is the last term (the number at the top).
    Thank you!
    • Thread Starter
    Offline

    7
    ReputationRep:
    (Original post by Science_help)
    since when is this c1??, what board are you doing ?
    Edexcel, haha this is from the solomon papers which is typically harder
    Offline

    10
    ReputationRep:
    Just sub in 10 and 30, your 'n' value is 30-10+1= 21, so 21/2(a+L)
    Offline

    2
    ReputationRep:
    (Original post by Mina_)
    Edexcel, haha this is from the solomon papers which is typically harder
    ahh adexcel, makes sense, in aqa we have this topic in c2, similar to how circle geometry is on c2 for edexcel, whereas its c1 for aqa?
 
 
 
  • 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
    Have you ever participated in a Secret Santa?
    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.