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# Edexcel A level S2 June 2009 Watch

1. The only thing I don't like is p.d.f of random variables. I'm not sure why but when you find K, say they find it as 3/16, when they plot it on the y axis on the graph sketch, it becomes 3/4 and I can't work out why.
2. I don't know how to integrate a PDF with several f(x) ranges for different x values, to a CDF...
Okay that doesn't make much sense.

f(x)=
{ 1/3, 0<=x<=1,
{ (8x^3)/45, 1<=x<=2,
{0, otherwise.

Specify fully the CDF F(x)

I'm not sure how to..
I would know if there was only one f(x) range, integrate f(x) with parameters x and the lower x value... right?

faaail.

3. (Original post by kat_ie)
I don't know how to integrate a PDF with several f(x) ranges for different x values, to a CDF...
Okay that doesn't make much sense.

f(x)=
{ 1/3, 0<=x<=1,
{ (8x^3)/45, 1<=x<=2,
{0, otherwise.

Specify fully the CDF F(x)

I'm not sure how to..
I would know if there was only one f(x) range, integrate f(x) with parameters x and the lower x value... right?

faaail.

ok you have to integrate each one, then put limits in with a random point, a, and then the lower limit of the group. then with the ones which have a fixed area before them, you have to add on the total area under the first section to the second one. this is because the cdf is the "probability up to this point"

ok i dont think that makes much sense
4. does anyone know if, when doing a binomial/poisson, and you write X~B etc, do you need to write "let X be the random variable that...." before you start? cause they do it sometimes on markschemes, but does that get marks?

and also, say in the second part they had a larger sample, would it be ok to still have it as X~B, cause they sometimes change the X to another variable in the mark schemes, and sometimes dont..
5. REALLY confused on this question...(June 2002 - hardish paper)
X-B(20, 0.4)
Find P(5<x<15)
So I took it as

1) - P(X<15) - which means P(X<= 15) and used the tables
2) - and the other side as P(X>5) which is 1-P(X<=5) and used the tables
Then subtracted the first from the second..

But the answer says you do P(X<=14) - (PX<=5)....how come there's no 1 minus for the 5? Confused!
6. The only questions i am getting wrong are to do with continuous random variables and cintinuous distributions. Any tips from anyone?
7. (Original post by fn_101)
REALLY confused on this question...(June 2002 - hardish paper)
X-B(20, 0.4)
Find P(5<x<15)
So I took it as

1) - P(X<15) - which means P(X<= 15) and used the tables
2) - and the other side as P(X>5) which is 1-P(X<=5) and used the tables
Then subtracted the first from the second..

But the answer says you do P(X<=14) - (PX<=5)....how come there's no 1 minus for the 5? Confused!
P(X<15) = P(X<=14) not 15, which is where you are going wrong. also if you picture the graph you need to do the
(prob that it is less than 15) - (prob less than 5) to get the prob that it is between the 2

hope that helps
8. (Original post by lazyman987)
does anyone know if, when doing a binomial/poisson, and you write X~B etc, do you need to write "let X be the random variable that...." before you start? cause they do it sometimes on markschemes, but does that get marks?

and also, say in the second part they had a larger sample, would it be ok to still have it as X~B, cause they sometimes change the X to another variable in the mark schemes, and sometimes dont..
my teacher always told us to write "x is r.v...." just in case, also it stops you from getting confused if you have it clear in your head first which way round you are doing it
9. (Original post by victoria13)
P(X<15) = P(X<=14) not 15, which is where you are going wrong. also if you picture the graph you need to do the
(prob that it is less than 15) - (prob less than 5) to get the prob that it is between the 2

hope that helps
Still a bit confused, sorry I get the 14 not 15 thing(did actually put that on paper hahaa) but does it not say P(X>5) so i would always think to do 1 minus, regardless of the graph
But if you do picture the graph, then how come it isn't P(X<=5) like the 14 thing?
10. (Original post by victoria13)
ok you have to integrate each one, then put limits in with a random point, a, and then the lower limit of the group. then with the ones which have a fixed area before them, you have to add on the total area under the first section to the second one. this is because the cdf is the "probability up to this point"

ok i dont think that makes much sense
ooh thanks
it makes perfect sense, i've been searching everything online all day trying to work it out.
was sort of obvious though.
cheeers
11. u want 2 find the values 6,7,8,9,10,11,12,13,14.
Therefore u want 2 take values less than 14, away from all values less than 5, leavin u with the values wanted. Hope tht makes sense.

Can anyone help me, i have no clue wot 2 do wen ur asked 4 a sampling distribution?
12. (Original post by thegreatest)
dude dont you complete the square!....with S2 just learn to always use the quadratic formulae (they hardly ever factorize in s2, so factorizing isnt an option usually).

I know quadratic formulae is alot of typing into the calculator but its routine, you dont have to spend time thinking about which method to use etc...

(also I see you are a muhammad ali fan = legend]
yeah i realise that now after doing a few papers.. quadratic equation fo sho' :P

yeah massive massive muhammad ali fan.. just coincidental that my name is Ali Mahmoud though :P
13. only thing that is remoteley challenging to me now is the sample frames and distribution things.. i need to nail them.. got around 8 hours of revision time left :P
14. (Original post by hparsons4)
u want 2 find the values 6,7,8,9,10,11,12,13,14.
Therefore u want 2 take values less than 14, away from all values less than 5, leavin u with the values wanted. Hope tht makes sense.

Can anyone help me, i have no clue wot 2 do wen ur asked 4 a sampling distribution?
haha i am just "lol"ing at how unprepared it seems we all are..

what you said is right btw (for fn_101)

also, for sampling distributions i think you just write the different combinations of samples you could have and then, say it asks for the samp. dist. of the mean, then you calculate the values of the mean, and then you find the probability of getting each mean, and times this by the number of combinations that give this mean.

make sense?? (usually people dont get my explanations haha)
15. (Original post by victoria13)
haha i am just "lol"ing at how unprepared it seems we all are..

what you said is right btw (for fn_101)

also, for sampling distributions i think you just write the different combinations of samples you could have and then, say it asks for the samp. dist. of the mean, then you calculate the values of the mean, and then you find the probability of getting each mean, and times this by the number of combinations that give this mean.

make sense?? (usually people dont get my explanations haha)
i dont feel unprepared.. im getting around 88% in the papers. i'm unprepared in the sense that i am targetting 100%.. if i graft for the rest of today im confident i can.
16. (Original post by Ali M)
i dont feel unprepared.. im getting around 88% in the papers. i'm unprepared in the sense that i am targetting 100%.. if i graft for the rest of today im confident i can.
i didnt mean everyone, it just seems a lot of people are probably stressing when they only get like 90 on a paper (me included) so most people want to get all the qs right, so they ask on here.

i expect most people on here will be aiming for very high score as well
17. (Original post by victoria13)
i didnt mean everyone, it just seems a lot of people are probably stressing when they only get like 90 on a paper (me included) so most people want to get all the qs right, so they ask on here.

i expect most people on here will be aiming for very high score as well
yeah.. true.. but TSRians don't represent the whole nation (hopefully) so the UMS might not be skewed too much i hope.. i think one year it was 68/75 for an A
18. (Original post by lazyman987)
does anyone know if, when doing a binomial/poisson, and you write X~B etc, do you need to write "let X be the random variable that...." before you start? cause they do it sometimes on markschemes, but does that get marks?

and also, say in the second part they had a larger sample, would it be ok to still have it as X~B, cause they sometimes change the X to another variable in the mark schemes, and sometimes dont..
You don't need to write that first thing you said, it never has marks allocated on the mark schemes.
It's more mathematically correct to change the variable; if you get into the habit of changing it, you won't think about it after a while. I don't think they penalise you for not changing it, but it makes things easier for the examiner to see where you've changed the distribution.
19. (Original post by Ali M)
yeah.. true.. but TSRians don't represent the whole nation (hopefully) so the UMS might not be skewed too much i hope.. i think one year it was 68/75 for an A
The grade boundries being so high brought my hopes down, I was getting around 90%, but I saw the boundries being nearly that high was crazy
20. (Original post by Ali M)
only thing that is remoteley challenging to me now is the sample frames and distribution things.. i need to nail them.. got around 8 hours of revision time left :P
What do you mean by sample frames and distrubtions things? like the definations or what?

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