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# Edexcel S2 - 27th June 2016 AM watch

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1. (Original post by iMacJack)
Is part a B = 23/6? I have a feeling I have done something wrong
Correct
2. (Original post by SeanFM)
It's to do with the critical region, the % significance and what value of k is most suitable (see from tables, trial and error)
yeah i thought of that but we're not given any value for Lambda so how would I know where to search
3. (Original post by imran_)
yeah i thought of that but we're not given any value for Lambda so how would I know where to search
The P(X=<9) column is probably the most useful.
4. [QUOTE=Bloom77;66112739]
(Original post by Bloom77)
Attachment 556741

This is what I did so far: Attachment 556743

But how do you know it's not the first function but it's the second one?

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i think with part 1 you realise that the first equation contains 0.6 of the overall probability and the second contains 0.4.
If you do F(3) you get 0.3. This means that the rest of the area of the CDF is 0.7. If you were to find P(3<X<a) = 0.642 you'd have to go from 3-4 in the CD which is (0.6 (the whole area under the first equation) - 0.3) = 0.3.

From that 0.3 you realise its less than 0.642 so that means you have to get the rest in the second equation. I would've done F(a) = 0.642-0.3
then solved the second equation=0.342

I dont know if its right but that's how i would've approached it. If you get two results then you choose the one above 3 I'll give it a go now.

edit: turns out the way i did it was wrong lol rip me. I looked at your method and it's right atm (i think). But I think you'd use the second equation for the reason i put above (it's somewhere in there lol). I did it your way and i got a=4.855?
5. (Original post by CherryTea)
Attachment 556857 Anybody know how to do 3b? It's from the june 2013 paper, I looked at exam solution but I still don't understand why you start off with p(x>n), how do we know that we should use it this way around?
That's from the context.

Remember that X is the number of games sold. So X>n means that more than n games are sold. And, from the question, you want that P(X>n) < 0.05 (in english, the probability of selling more than n games, (because they only have n) should be less than 0.05).
6. (Original post by SeanFM)
The P(X=<9) column is probably the most useful.
Sorry im confused
7. (Original post by Superbubbles)
Need help on 2015 paper q3b anyone know??
the first equation is a quadratic, and the second equation is a linear line. as the second equation has a negative gradient as the coefficent of X is - this means that the highest point on the PDF would be where X=2 as this is the first point on the linear equation. 6 would be the lowest point. hope this makes sense
8. Does anyone have a full list of definitions we should know?
9. (Original post by SeanFM)
Correct
K = -936/391 ?

Again I don't think I'm right xD
10. (Original post by imran_)
Sorry im confused
There isn't much else to say it is a very difficult question (the replacement paper, I presume) but you've got all the information there and you just need to scan through the different values of lambda and look for the most appropriate one.
11. (Original post by iMacJack)
K = -936/391 ?

Again I don't think I'm right xD
Afraid not - k is a whole number - actually, let me double check that. I think I've messed it up

Yes - k should be a whole number. Phew.
12. (Original post by SeanFM)
Afraid not - k is a whole number - actually, let me double check that. I think I've messed it up

Yes - k should be a whole number. Phew.
Only other value I get (when I do my calcs properly) is k = 18/23

Maybe I'm doing it wrong though?

I'm integrating between the limits and then just setting = 1?
13. (Original post by SeanFM)
The P(X=<9) column is probably the most useful.
I would have thought that it would be P(X<=8) because CR is X>=9?
14. (Original post by iMacJack)
Only other value I get (when I do my calcs properly) is k = 18/23

Maybe I'm doing it wrong though?

I'm integrating between the limits and then just setting = 1?
integrating what between which limits, and why? (you do have to set something equal to 1, so you are heading in the right direction)
15. (Original post by r3h4f)
I would have thought that it would be P(X<=8) because CR is X>=9?
(Original post by imran_)
x
Yes, that is correct - sorry. i didn't put a huge amount of thought into it.
16. (Original post by SeanFM)
integrating what between which limits, and why? (you do have to set something equal to 1, so you are heading in the right direction)
F(B) = 1

And integrating both parts of f(x) between their limits and setting = to 1 (because total prob = 1) so then I can find the value of K?
17. Posted from TSR Mobile

Could someone quickly explain to me how to solve 4c Jan 2013?
Attached Images

18. (Original post by iMacJack)
F(B) = 1

And integrating both parts of f(x) between their limits and setting = to 1 (because total prob = 1) so then I can find the value of K?
Correct (though if you sketch the half of the distribution that you do know, it may help to..save some integrating, is all I will say).

Perhaps something is going wrong in the calculation or writing things down?
19. (Original post by kosvengali)
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Could someone quickly explain to me how to solve 4c Jan 2013?
Add five to both sides of the inequality and work it out? Take care of the 8 in the inequality though, as the maximum value is 6, so adjust the inequality taking note of this
20. (Original post by kosvengali)
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Could someone quickly explain to me how to solve 4c Jan 2013?
If -3 < X - 5 < 3 then a < X < b where a =... and b =....

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