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    can someone pls explain part d???? what i did was find the sum of the first 3 (1+(-1/2) +1) then multiplied it by 100/3 and i got an answer of 50 . why is this wrong?

    plus this is not arithmetic so how did it use that rule
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    Write out the first 4 terms, notice anything?
    The sequence repeats every 4 terms, 1,-1/2,1,-1/2 ...
    If we are summing the first 100 terms then how many lots of this sequence is there?
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    (Original post by pondsteps)
    can someone pls explain part d???? what i did was find the sum of the first 3 (1+(-1/2) +1) then multiplied it by 100/3 and i got an answer of 50 . why is this wrong?

    plus this is not arithmetic so how did it use that rule
    the first 3 terms as they are in the sequence don't repeat i.e in this sequence the first 3 terms goes 1, -0.5, 1

    when you add the first 3 terms multiply by 100 then divide by 3 you are assuming the sequence goes like this

    [1,-0.5,1,][1,-0.5,1] etc etc
    because you've assumed the firts 3 terms repeat one after another, but they don't

    an arithmetic sequence is one where the difference between the terms is constant
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    (Original post by pondsteps)
    can someone pls explain part d???? what i did was find the sum of the first 3 (1+(-1/2) +1) then multiplied it by 100/3 and i got an answer of 50 . why is this wrong?

    plus this is not arithmetic so how did it use that rule
    There should either be a general pattern in terms or it may be an arithmetic

    What is  X_1, X_2 & X_3 ?
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    (Original post by EqualRights)
    There should either be a general pattern in terms or it may be an arithmetic

    What is  X_1, X_2 & X_3 ?
    fist term second term and third term
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    (Original post by thefatone)
    the first 3 terms as they are in the sequence don't repeat i.e in this sequence the first 3 terms goes 1, -0.5, 1

    when you add the first 3 terms multiply by 100 then divide by 3 you are assuming the sequence goes like this

    [1,-0.5,1,][1,-0.5,1] etc etc
    because you've assumed the firts 3 terms repeat one after another, but they don't

    an arithmetic sequence is one where the difference between the terms is constant
    ohhh... thank u sooo much!! xxx
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    (Original post by thefatone)
    fist term second term and third term
    no omg, I meant as in values haha
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    (Original post by EqualRights)
    There should either be a general pattern in terms or it may be an arithmetic

    What is  X_1, X_2 & X_3 ?
    okay so i can use the rule 1/2 (n) (a+L) even if the sequence is not arithmetic??
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    (Original post by pondsteps)
    okay so i can use the rule 1/2 (n) (a+L) even if the sequence is not arithmetic??
    No. That only works for arithmetic sequences.
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    (Original post by B_9710)
    No. That only works for arithmetic sequences.
    umm but the above example is not arithmetic ... and i thought it was only for arithmetic
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    (Original post by pondsteps)
    umm but the above example is not arithmetic ... and i thought it was only for arithmetic
    Exactly, so you can't use that formula.
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    (Original post by Zacken)
    Exactly, so you can't use that formula.
    lol but the markscheme said accept and it worked 😁
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    The first 2 terms sum to 1/2 and there are 50 of them so the full sum is 25.
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    (Original post by pondsteps)
    lol but the markscheme said accept and it worked 😁
    Yes, the original sequence isn't arithmetic but you can sum every two terms and treat it as an arithmetic (common difference 0) sequence with 50 terms.
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    (Original post by EqualRights)
    no omg, I meant as in values haha
    well first term is given as 1 then use the reccurence relation to fin the next 2 ^-^
 
 
 
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