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    Im really stuck on question 17 section 2B in the hienemann modular mathematics book...I've tried so many different ways of trying to solve it but i can't get the right answer. It's probably really basic..but i'm abit slow

    Find the sum of all the even numbers between 2 and 200 inclusive, excluding those which are multiples of 3.

    I tried finding the sum of the even numbers between 2 and 200, then subtracting the sum of the multiples of 3 between 2 and 200. but thats wrong.:confused: Im so so confused.
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    The sum you want looks like:

    2+4+ 8+10+ +14+16+ 20+22+ 26+...

    Are you sure you excluded the right multiples of three?

    [If this hint doesn't help, post your working].
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    umm this is my working out: sum of [2r] between r=1 and n=200 subtract the sum of [2r] between r=1 n=1....then i subtracted (the sum of [3r] between r=1 n=200 subract the sum of [3r] between r=1 and n=1
    ) i'm so sorry its abit messy...and wrong...
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    Look at the sum I posted and compare with the straightforward sum 2+4+6+8+10+12+14+16+18+20+22+24+ ...

    Which numbers are missing?

    Do you want to post your answer for the sum of the even numbers between 2 and 200 as well; I'm not convinced you're using the right values.
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    i've missed out '2'. Im really bad at these questions. i thought that if i wanted to find the sum of the even numbers i would use 2r....im really clueless. I don't know how to approach these kinds of questions.
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    to find the sum of the simple series 2+4+6+8+10... i would use the sum of 2r using r=1 and n=200 but i don't know how to exclude multiples of 3
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    If you're stuck on a question like this, one thing to do is to reduce the scale to something you can deal with.

    E.g. find the sum of all the even numbers between 2 and 20 inclusive, excluding those which are multiples of 3.

    So this is 2+4+8+10+14+16+20 = 2+4+6+8+10+12+14+16+18+20 - (6+12+18)

    This way you can check your answer (and indeed the two "parts" of your answer; the "sum of the even numbers" and "minus the sum of the excluded numbers") with a fair chance of finding out what's wrong if it doesn't work.

    Edit: You say "this is what I would do", but you don't actually do it, or even post your answer. Which makes it impossible to try to work out if/where you are going wrong.
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    ok thanks for your help, i'll try it on a smaller scale first hopefully i can then see where im going wrong.
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    (Original post by venessa_carter)
    to find the sum of the simple series 2+4+6+8+10... i would use the sum of 2r using r=1 and n=200 but i don't know how to exclude multiples of 3
    I'm not sure whether DFranklin spotted this and was hoping you'd work it out yourself (if so, sorry :p:), but you're not meant to do it like that - if you want the sum 2+4+6+8+...+200, you need to sum 2r from r=1 to r=100. (Because if r = 100, 2r = 200.)
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    (Original post by generalebriety)
    I'm not sure whether DFranklin spotted this and was hoping you'd work it out yourself (if so, sorry :p:), but you're not meant to do it like that - if you want the sum 2+4+6+8+...+200, you need to sum 2r from r=1 to r=100. (Because if r = 100, 2r = 200.)
    I spotted it, but because she brought in a mysterious 'n' term without explaining it, I wasn't completely sure she was making that mistake. But that's why I asked her both to say what she got for the sum of the even numbers and to show her working - neither of which she did, of course... :rolleyes:
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    thanks for your help! much appreciated. I was really confused at first but noticed i made loads of mistakes...i'll have to practice theses types of questions more.
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    Guess so.

    Did you get that question out in the end?
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    I've just got to this question a I'm teaching myself FP1 - and I remembered this thread.

    This is what I did - I worked out the sum from 2-200 pretty simply, then I used a similar method for the even multiples of 3.

    If you're working out all the multiples of 3 then you sum 3r between 1-66ish.

    But even multiples of 3 will work under the patter 6r from 1-33.

    Do this, subtract and you get the nswer 6734.

    Hope this helps.
 
 
 
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