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# FP1 series - desperate help!:( watch

1. 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. Im so so confused.
2. 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].
3. 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...
4. 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.
5. 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.
6. 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
7. 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.
8. ok thanks for your help, i'll try it on a smaller scale first hopefully i can then see where im going wrong.
9. (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 ), 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.)
10. (Original post by generalebriety)
I'm not sure whether DFranklin spotted this and was hoping you'd work it out yourself (if so, sorry ), 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...
11. 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.
12. Guess so.

Did you get that question out in the end?
13. 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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