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# Official Edexcel S3 thread Wednesday 20 May 2015 Morning [6691] watch

1. Is anyone here going to write their answers down here after the exam? Would appreciate it if you did as Arsey doesn't do s3 and i really need to know if i got them correct.
2. (Original post by Damien_Dalgaard)
Can anyone kindly help me out on this question please

Random variables

Ex 1A Question 10

https://771a1ec81340d97ae9ed29694f73...XZCVmc/CH1.pdf

The units of variance are in cm2 why isnt this being converted.
But when they use variance in the equations, they square root it (i.e using s.d which has the units cm)?
3. (Original post by STATER)
Not quite sure what you're asking.

You are looking for a length of bookshelf where the probability is greater or equal to 0.99. So you find a Z value that corresponds to this (-2.3263). Their working is unnecessary, and leads to confusion.
Then simply sub it into the normal distribution standardisation formula to find x. I tried to attach my working but its not uploading.
Thank you so much for the clear explanation, I really hate these books, found another error :/
4. (Original post by xKay)
Is anyone here going to write their answers down here after the exam? Would appreciate it if you did as Arsey doesn't do s3 and i really need to know if i got them correct.
Xin Xang is going to get 100UMS

Just ask him (I think that he is a him)

I must be the only person who never looks at solutions having given the paper, not much you can do after smh

(Original post by rayquaza17)
But when they use variance in the equations, they square root it (i.e using s.d which has the units cm)?
No since it is not the centimetres being squared only the metres which means that cm^2 = (1*10^-2) * (1*10^-2) = 1*10^-4 m.

ffs you are actually right since the square root of 1 cm would be 1*10^-2 m or in other words 1 cm, with the relevant normal value associated with it ceteris paribus.

newb mistake on my part.

Thanks.
5. When writing out the definitions do you try to write it in words or mathematically?

As in do you write for the Central Limit Theorem "for a large n, follows ", or "for a large sample, the sample mean follows a normal distribution around the population mean with variance "?
6. Can anyone suggest a source for difficult practice questions/difficult papers?
7. (Original post by Karoel)
When writing out the definitions do you try to write it in words or mathematically?

As in do you write for the Central Limit Theorem "for a large n, follows ", or "for a large sample, the sample mean follows a normal distribution around the population mean with variance "?
Better in words but I tend to add the mathematical bit on at the end just in case. Marks are given for wordy definitions though, it seems.
8. (Original post by Karoel)
When writing out the definitions do you try to write it in words or mathematically?

As in do you write for the Central Limit Theorem "for a large n, follows ", or "for a large sample, the sample mean follows a normal distribution around the population mean with variance "?
You can do it mathematically as long as you define every term. So you need to clearly state what n, and are.
9. (Original post by STATER)
Can anyone suggest a source for difficult practice questions/difficult papers?

June 14 ials

http://www.physicsandmathstutor.com/...dexcel-papers/

Review exericses eddecel book
10. (Original post by Damien_Dalgaard)
June 14 ials

http://www.physicsandmathstutor.com/...dexcel-papers/

Review exericses eddecel book
I find the review exercises to be no harder than edexcel papers. Saving the IAL for the final few days .
11. (Original post by STATER)
I find the review exercises to be no harder than edexcel papers. Saving the IAL for the final few days .
I haven't really looked at the RE just based it off what I did on previous past papers.

12. (Original post by STATER)
Not quite sure what you're asking.

You are looking for a length of bookshelf where the probability is greater or equal to 0.99. So you find a Z value that corresponds to this (-2.3263). Their working is unnecessary, and leads to confusion.
Then simply sub it into the normal distribution standardisation formula to find x. I tried to attach my working but its not uploading.
Sorry to bother you but I am still a tad unsure, I am a bit frazzled in how you can you use the z value +/-2.3263 to work out what 'x' is.
13. do we have to know the proofs for the unbiased estimators of variance and mean?
14. (Original post by rayquaza17)
Could you post the mark scheme? The bias is definitely E[theta]-theta.
Thanks for the detailed reply. This is actually from last year's IAL paper, although I don't see why they would define bias differently for that syllabus. Question 2.

https://869d950bf149eefd5cd2652b4001...%20Edexcel.pdf

(Original post by rayquaza17)
I would go for fractions tbh, it's more accurate.
But someone above says to use 4dp? If thats what the mark schemes suggest you should do, then do that. I don't see how they can justify wanting decimals over fractions though!!
I suppose usually they'll have awrt for a number to about 2dp for the test statistic so rounding all the previous numbers to 4dp should easily result in an answer accurate to that order, but writing fractions still shows you know the method so I just don't see what's wrong with it
15. (Original post by 13 1 20 8 42)
Thanks for the detailed reply. This is actually from last year's IAL paper, although I don't see why they would define bias differently for that syllabus. Question 2.

https://869d950bf149eefd5cd2652b4001...%20Edexcel.pdf

I suppose usually they'll have awrt for a number to about 2dp for the test statistic so rounding all the previous numbers to 4dp should easily result in an answer accurate to that order, but writing fractions still shows you know the method so I just don't see what's wrong with it
The answer is the alpha/2, but I think they were just being generous and letting people have the mark for -alpha/2.
16. (Original post by Damien_Dalgaard)
Sorry to bother you but I am still a tad unsure, I am a bit frazzled in how you can you use the z value +/-2.3263 to work out what 'x' is.
My mistake, it's not -2.3263, it's positive.

(Original post by angelwattings)
do we have to know the proofs for the unbiased estimators of variance and mean?
Yes, it came up in 2014 for one mark.
Attached Images

17. (Original post by xKay)
Is anyone here going to write their answers down here after the exam? Would appreciate it if you did as Arsey doesn't do s3 and i really need to know if i got them correct.
I can probably grab hold of the S3 paper after the exam and send them through, if anyones interested?

P.S. I work in a college
18. (Original post by kingmathematica)
I can probably grab hold of the S3 paper after the exam and send them through, if anyones interested?

P.S. I work in a college
yes that would be really helpful
19. (Original post by xKay)
Is anyone here going to write their answers down here after the exam? Would appreciate it if you did as Arsey doesn't do s3 and i really need to know if i got them correct.
I am happy to post the answer, but not guaranteed 100% though. Would be nice if i got all right
20. (Original post by Damien_Dalgaard)
June 14 ials

http://www.physicsandmathstutor.com/...dexcel-papers/

Review exericses eddecel book
This IAL paper is the hardest so far... I hope this year will be easier than that :/ need the grades for uni lol

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