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Mathematics: Post your doubts here!

Jiyad Ahsan

Jiyad Ahsan

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Alice123 said:
Same year bi n ii..... HELP!!! n also Jiyad Ahsan if u arent busy
Click to expand...
okay so if order doesnt matter its 3876
and since 4 are selected you multiply by 4!
=93024
another way to explain it is that
nCr = n! / r! (n-r)! - - - and nPr = n!/ (n-r)!
so if you cancel it out
nPr = nCr x r!

and i'm solving b(ii) right now,will post it in a bit
 
Jiyad Ahsan

Jiyad Ahsan

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Alice123 said:
6b... missed the 6...
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okay part (ii) since there are 4 groups each with 3 possessions so you have
3! x 3! x 3! x 3! (or 3!^4 do that the 3 possessions can be arranges within their group) multiplied by 4! (so the 4 groups can be arranged as well)
3!^4 x 4!
=31104
 
Jiyad Ahsan

Jiyad Ahsan

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Zenia ZZ said:
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Mathematics (9709)/9709_w12_qp_62.pdf
Q5 (b ii and iii) please !
Jiyad Ahsan
Click to expand...

part (i)
12P8
=19958400

part (ii)
so frances and mary do not sit together,
just find out the number of ways in which they DO sit together and subtract from the total
if they sit together then 11P7 x 2P2 - - - - (11P7 because since they sit together you count them as one unit, 2P2 because they can arrange themselves in their group, sit left or right)
11P7 x 2P2
=3326400
this is when they do sit together, so subtracting from total ( part (i))
19958400 - 3326400
=16632000

part (iii)
i'll just upload the working for it in a bit
 
Jiyad Ahsan

Jiyad Ahsan

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ok everyone, im outta here, i have physics as well tomorrow
Goodluck everyone, insha'Allah you'll do great :)
 
hassanhijazi1995

hassanhijazi1995

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Anika Raisa said:
Um i m nt clear abt ur question bt i m assuming dat u r talking about Normal approximation...

So when u u use it when u have calculated probability using bionomal or ur mean n variance using np n npq respectively!!
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am sure about this...
when they give you in the question the mean and the S.D no need to add 0.5 or subtract 0.5

but when they give you the probabilty of success and failure and u need to calculat the mean and S.D. here you will +- 0.5
 
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PhyZac said:
This is very tricky question. Took me time to understand, anyway,


Alice123
you are right about (20+12<x<20-12)

now you have to imagine the graph in your head, and you have to come into a conclusion that it is symmetrical.

Now imagine the z of 20+12 that is 32 is x
So the z of 20-12 that is 8 will be a negative number -x

Both of them are x but one is negative, because they are the same distance from the mean

Let the probability of x is p
so the probability of -x is 1-p

we know that difference of the two probability is 0.94

so p - (1-p) = 0.94
p - 1 + p = 0.94
2p = 0.95 + 1 = 1.94
p = 0.97

Now is p is 0.97 z is from the table, 1.882 (or 1.881)

now continue normally

1.882 = 32-20 / sd
sd = 12/1.882
=6.38
Click to expand...
you mean to say i will have to do all this for so little marks :O
 
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aleezay

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In p6 when scaling variables.. rq when the give u (x-50)^2 and ask you to calculate s.d of x... what formula do we use? S.dy = aS.dx or S.dy = |a|S.dx
where a is x's coefficient and y is x-50..
 
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aleezay

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Zenia ZZ said:
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Mathematics (9709)/9709_s12_qp_61.pdf
please help me with(question 6 ii )
and for question 7 (a ii ) 7! x 7! (is for arranging the friends and the partners ) but we are multiplying by 2 because either the friends could be first or the partners ? am i right ?? i want to confirm .
Click to expand...
Yes you are.. for your first q, let me give you the main idea. If you still have trouble solving it, ill post the whole working :)
more than an s.d from the mean means: P(X<mean-s.d) + P(X>mean+s.d).. youll get the probability. Since expected number is no of trials x probability.. youll multiply your probability with 1000. Simple :)
PS= In such qs, round off your answer to 3sf and not a whole no, even though the q asks you to give the expected number of birds :)
 
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