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# Edexcel S3---9/06/05 watch

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1. (Original post by Spenceman_)
Var [aX] = a2 Var [X]
That sounds promising. I was trying to remember the rule - it would have helped if I got pen and paper out and to work from the definition.
2. lol mine is
P1-100
P2-94
P3-91
P4-86
M1-95
S1-75 >> retaking
good luck im gona try n get down to some more past papers
3. (Original post by Spenceman_)
Just for the record I mean the following: -

Var [3X] = 9 Var [X]

BUT Var[X1 + X2 + X3 ] = 3 Var [X]

...because they are separate observations of the variable as opposed to one observation getting scaled.

So say it said...time taken for a car to drive one mile is distributed X~ N(10,100)

What's the probability it takes less then 35 minutes for a car to drive the three mile journey from here to school. You are dealing with X1 + X2 + X3 ~N(30,300) and not 3X ~ N(30,900).
thts probably where i was confused
thanx!
4. (Original post by Spenceman_)
Just for the record I mean the following: -

Var [3X] = 9 Var [X]

BUT Var[X1 + X2 + X3 ] = 3 Var [X]

...because they are separate observations of the variable as opposed to one observation getting scaled.

So say it said...time taken for a car to drive one mile is distributed X~ N(10,100)

What's the probability it takes less then 35 minutes for a car to drive the three mile journey from here to school. You are dealing with X1 + X2 + X3 ~N(30,300) and not 3X ~ N(30,900).
yeh for example if a question says the weight of a women follow a normal (8,10). 3 women go into a lift. What is the distribution of there weight it would be

W~N(8+8+8,10+10+10)

but if it said a woman goes into the lift whose weight is three times that of a normal woman then

w~N(8+8+8, 9x10)
5. My P1 paper from last year contains the statement 5 x 2 = 20. Hmmm. It didn't deserve to be scaled to 100.
6. (Original post by hajira)
lol mine is
P1-100
P2-94
P3-91
P4-86
M1-95
S1-75 >> retaking
good luck im gona try n get down to some more past papers
me too

good luck tmmrw!!
7. (Original post by BLUREMI)
yeh for example if a question says the weight of a women follow a normal (8,10). 3 women go into a lift. What is the distribution of there weight it would be

W~N(8+8+8,10+10+10)

but if it said a woman goes into the lift whose weight is three times that of a normal woman then

w~N(8+8+8, 9x10)
ahh there goes my concern.. but its weird
i hope im able to spot that!!
8. hajira do you have the mock s3 answers would be grateful if you could send it 2me
9. (Original post by kevins)
hajira do you have the mock s3 answers would be grateful if you could send it 2me
ahh.. i have the mark scheme for that but i jus came on the computer now! if u r around tell me ur email address n il send it

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Updated: June 9, 2005
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