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# How was Edexcel S2? watch

1. 4c) - I got 0.122, so yep i agree
4d) - i got 0.0024, yep
4e) mean = 2, variance = 1.8 - yep
3) 0.0713.

same for those as well.

What did people get for the F(x)

I got x/3 for 0<x<1

and 2/45x^4 for the 1<x<4 tho people seemed have a constant added on the end :O

∫8/45x^3 = 2/45x^4 doesn't it?
2. (Original post by BlueAngel)
For d) I used normal.

Could someone tell me if I was right or wrong?
i did the same

does anyone remember the answers to q 5?
3. (Original post by Neutrinohunter)
∫8/45x^3 = 2/45x^4 doesn't it?
You had to take into account that F(x) is cumulative and therefore you had to add the maximum value from the previous bit of the distribuiton (i.e. 1/3) and integrate between the lower limit (1) and the max possible value (infinity or x)....at least I think thats correct
4. you needed a 1/3 constant at the end
5. Oh well i still seemed to get the right answer for the median, can't see how i can lose about 15 marks or whateva on the paper to not get an A but i don't wanna be too confident, cos many people found the paper rele easy
6. (Original post by Leekey)
That was either the easiest S2 paper I have ever sat or it was the hardest. I think I did everything correctly but I kept waiting for a difficult git of a question and it just didn't seem to happen (think I might have failed due to the fact that I found it too easy)....
hey leekey... told ya it wont be a problem... how did you find your resits? u should get your A then
7. (Original post by clare_m)
indeed, you are correct

Are you talking to me?
8. (Original post by S1M)
hey leekey... told ya it wont be a problem... how did you find your resits? u should get your A then

Wassup S1M, it was 'easy' init..... LOL
9. (Original post by S1M)
hey leekey... told ya it wont be a problem... how did you find your resits? u should get your A then
God I hope I get at least a B. I've put EVERYTHING I have into maths this time round and I just pray its going to get me into uni. The resits were all fairly decent (except S1 because of my inability to so any stats), I think my P1 mark will boost my grade and make up for a shocker in P3 or S2 though (hopefully not both)!!!

How have you gone on today?
10. (Original post by Leekey)
God I hope I get at least a B. I've put EVERYTHING I have into maths this time round and I just pray its going to get me into uni. The resits were all fairly decent (except S1 because of my inability to so any stats), I think my P1 mark will boost my grade and make up for a shocker in P3 or S2 though (hopefully not both)!!!

How have you gone on today?
Yeh, i thought it was ok too... other than that 1st question i think ill get most of the marks... so you think your 3 days revision payed off then?

(Original post by DaKe)
Wassup S1M, it was 'easy' init..... LOL
lol... shut up m8...
11. For the questions where you had X~B(100,0.1) and you were supposed to approximate it to find P(X > 15) or something, should it have been approximated as normal or poisson?

I thought you could do poisson as "n is large and p is small"
or you could do normal as np > 5 and n(1-p) > 5

so which should've been used? or would either have been ok?
12. (Original post by S1M)
so you think your 3 days revision payed off then?
Based on the fact that 3 days ago I couldn't do probability density functions and my hypothesis testing was shakey and now I'm feeling pretty decent about the exam, I think that it worked pretty well yeah!!!
13. I'm wondering if that was too easy to be true! Especially the first 6 qus. I tried to remember my answers, wrote some down and I think I got:
1) [definitions]
2) a: 0.54, b:1.5, c:2.083
3) don't remember
4) a:Binomial rules, b:X~B(70, 0.1), c:0.1216, d:0.0024, e(x)=2 & Var(x)=0.8, f:0.0487 (or something like that)
5) a:critical value def, b:0.9907, c:0.947, d:0.0733
6) a: X~Po(0.05), b:0.251, c:0.0045, d:0.1562
7) a:1.269, b:x/3 & something involving 45, c:1.831, d: negitive skew.

If any don't look right, then I may have either totally effed it up or remembered them wrong!
Did anyone get the same?
14. (Original post by davedave)
For the questions where you had X~B(100,0.1) and you were supposed to approximate it to find P(X > 15) or something, should it have been approximated as normal or poisson?

I thought you could do poisson as "n is large and p is small"
or you could do normal as np > 5 and n(1-p) > 5

so which should've been used? or would either have been ok?
i used poisson for that... i was unsure which one... because both seemed feasible imo... but as i noticed n was large... so i went for poisson... not sure if "p is small" enough though... thats why i had my doubts... coz that means the mean is 10 and the variance is 9... for poisson mean is equal to variance... so as thats quite a big % difference between 9 and 10... i thought it could be normal too... but i ended up doing poisson anyway... well i tryed the normal quickly on my calculator and it gave a slightly lower answer anyway... so if poisson is wrong hopefully we should still get some marks...
15. (Original post by davedave)
For the questions where you had X~B(100,0.1) and you were supposed to approximate it to find P(X > 15) or something, should it have been approximated as normal or poisson?

I thought you could do poisson as "n is large and p is small"
or you could do normal as np > 5 and n(1-p) > 5

so which should've been used? or would either have been ok?
I used Poisson cause I prefer it, but I double checked with the Normal & only got 0.0001 difference, so I don't think it matters. It works for both.
16. MATHS IS OVER!! HOORAH!!

So anyway, big stress now. I'm trying to get last year's mark boundary thingys off the edexcel site, but i dont think i have adobe acrobat (?) on my computer - that or it wont work.

can anyone have a look and post them up for p2-3 and s1-2?

i really dont think ive got my a overall....

this paper was fine though - ive just realised i got the skew bit wrong - i thought mode<mean<median but its mode<median<mean so thats 2 marks gone. im not hunting for any more missed marks - ill get upset
17. When approximating from a Binomial to a Normal it is best if the probability is about 0.5 to ensure symmetry (I think anyway)!!! Since 0.1 wasn't anywhere near 0.5, I decided to use a poisson.
18. how did u work out the median?

this is wot i did:

f(x) = 1/3 0<=x<1
= 8x^(3)/45 1<=x<=2
= 0 elsewhere

the median will be in the second bit

so integrate with limits m and 1 making it equal to a half

rearrange to make x subject

and you should get a quartic root

someone validate me
19. (Original post by Louise87)
I used Poisson cause I prefer it, but I double checked with the Normal & only got 0.0001 difference, so I don't think it matters. It works for both.

np>10 so use normal approximation

in the exam u usually use poisson if p is less than 0.1

0.1 is fairly central
20. (Original post by mabs)
how did u work out the median?

this is wot i did:

f(x) = 1/3 0<=x<1
= 8x^(3)/45 1<=x<=2
= 0 elsewhere

the median will be in the second bit

so integrate with limits m and 1 making it equal to a half

rearrange to make x subject

and you should get a quartic root

someone validate me
I remember something like X^4 = 4.74 of something like that....

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