• Congratulations to the Class of 2024 on your results!
    Let us know how you went here
    Got a question about your uni preferences? Ask us here

HSC 2015 MX2 Permutations & Combinations Marathon (archive) (4 Viewers)

Status
Not open for further replies.

rand_althor

Active Member
Joined
May 16, 2015
Messages
554
Gender
Male
HSC
2015
Re: 2015 permutation X2 marathon

Is the aim here to block out my questions?
Sorry, that's not my intention. I figured I'd forget to post this one if I didn't now. I did attempt your question, but I have no idea where to proceed after finding the discriminant. Your hint of using integration makes me think it is a portion of an area bounded by a curve and maybe the x-axis representing the values for which the quadratic has real roots, but I'm not sure if that's correct. Even if it is, I don't know how to proceed.
 

InteGrand

Well-Known Member
Joined
Dec 11, 2014
Messages
6,109
Gender
Male
HSC
N/A
Re: 2015 permutation X2 marathon


Now my question again:

p and q are real numbers.
p is randomly chosen on the interval -apa
q is randomly chosen on the interval 0 ≤ qb

What is the probability that the equation x² + px + q = 0 has real roots ?


At its core, this is not hard. It involves discriminant and area under a curve.
There is just one hitch.

Haven't tried the Q. much, but to find the probability that one random variable is less than another, wouldn't we essentially need to use double integration (at least, this is one way of doing it)?
 

Drsoccerball

Well-Known Member
Joined
May 28, 2014
Messages
3,650
Gender
Undisclosed
HSC
2015
Re: 2015 permutation X2 marathon

Question I saw: In a chess tournament, n women and 2n men participated, and each one of them played only one game with everybody else. The ratio of the number of games won by women to the number of games won by men is 7:5. Find the number of men that participated in the tournament if no game was over in a tie.
Answer: 6 men
Are you sure the answers 6 men? If theres 6 men that means n=3. Which also means there is 9 people. If each player plays only 1 person there would be 1 person that doesnt play...?
 

kawaiipotato

Well-Known Member
Joined
Apr 28, 2015
Messages
463
Gender
Undisclosed
HSC
2015
Re: 2015 permutation X2 marathon

Are you sure the answers 6 men? If theres 6 men that means n=3. Which also means there is 9 people. If each player plays only 1 person there would be 1 person that doesnt play...?
I think when they said they play with everyone once, it's similar to a group of people all giving handshakes to everyone only once. So no-one is left out.
 

Drsoccerball

Well-Known Member
Joined
May 28, 2014
Messages
3,650
Gender
Undisclosed
HSC
2015
Re: 2015 permutation X2 marathon

I think when they said they play with one person at least once, it's similar to a group of people all giving handshakes to everyone only once. So no-one is left out.
I now think it means everyone plays everyone once.
 

braintic

Well-Known Member
Joined
Jan 20, 2011
Messages
2,137
Gender
Undisclosed
HSC
N/A
Re: 2015 permutation X2 marathon

Haven't tried the Q. much, but to find the probability that one random variable is less than another, wouldn't we essentially need to use double integration (at least, this is one way of doing it)?
p² - 4q must be positive.
That is a region on the p-q Cartesian plane.

The restrictions on p and q define a rectangular box on the Cartesian plane.
So you basically just need to find the fraction of the box than contains the above region.

The hitch is the the corner of the box doesn't have to lie on the parabola. So you need two cases - corner inside and corner outside the parabola.


The reason for asking this question is that it leads on to another question which is strongly based on a question in a past Ext 2 HSC exam (it wasn't on probability, but the technique is the same).
 

braintic

Well-Known Member
Joined
Jan 20, 2011
Messages
2,137
Gender
Undisclosed
HSC
N/A
Re: 2015 permutation X2 marathon

Sorry, that's not my intention. I figured I'd forget to post this one if I didn't now. I did attempt your question, but I have no idea where to proceed after finding the discriminant. Your hint of using integration makes me think it is a portion of an area bounded by a curve and maybe the x-axis representing the values for which the quadratic has real roots, but I'm not sure if that's correct. Even if it is, I don't know how to proceed.
Sorry - I thought you were the one who also posted the previous question.

Anyway - in your question is there an assumption that men and women have equal ability?
 

Paradoxica

-insert title here-
Joined
Jun 19, 2014
Messages
2,556
Location
Outside reality
Gender
Male
HSC
2016
Re: 2015 permutation X2 marathon

p² - 4q must be positive.
That is a region on the p-q Cartesian plane.

The restrictions on p and q define a rectangular box on the Cartesian plane.
So you basically just need to find the fraction of the box than contains the above region.

The hitch is the the corner of the box doesn't have to lie on the parabola. So you need two cases - corner inside and corner outside the parabola.


The reason for asking this question is that it leads on to another question which is strongly based on a question in a past Ext 2 HSC exam (it wasn't on probability, but the technique is the same).
But p^2 - 4q positive is the same as the probability that 2logp > log4 + logq? Log is concave, so that won't change the probability.
 

InteGrand

Well-Known Member
Joined
Dec 11, 2014
Messages
6,109
Gender
Male
HSC
N/A
Re: 2015 permutation X2 marathon

But p^2 - 4q positive is the same as the probability that 2logp > log4 + logq? Log is concave, so that won't change the probability.
That log thing would give half the probability, as it is the case where p > 0.
 

braintic

Well-Known Member
Joined
Jan 20, 2011
Messages
2,137
Gender
Undisclosed
HSC
N/A
Re: 2015 permutation X2 marathon

But p^2 - 4q positive is the same as the probability that 2logp > log4 + logq? Log is concave, so that won't change the probability.
I haven't thought about this much yet, but my suspicion is that the moment you take logs you are no longer picking numbers with equal probability over the intervals specified.

We are not simply solving p^2 >4q - we are finding points in a particular region that satisfy that inequality. We are changing the shape of the region by taking logs.

Perhaps seanieg could confirm that.
 
Last edited:

braintic

Well-Known Member
Joined
Jan 20, 2011
Messages
2,137
Gender
Undisclosed
HSC
N/A
Re: 2015 permutation X2 marathon

Still haven't had time to think about it properly. But you talk about THE result. There are two different results depending on the relative values of a and b.
 

InteGrand

Well-Known Member
Joined
Dec 11, 2014
Messages
6,109
Gender
Male
HSC
N/A
Re: 2015 permutation X2 marathon

Still haven't had time to think about it properly. But you talk about THE result. There are two different results depending on the relative values of a and b.
No I wasn't referring to that question, by 'the result' I meant P(A) = P(B). (And these were unrelated to the "a" and "b" of the question.)
 
Last edited:

seanieg89

Well-Known Member
Joined
Aug 8, 2006
Messages
2,662
Gender
Male
HSC
2007
Re: 2015 permutation X2 marathon

Sure, P(A > B)=P(log(A) > log(B)) for positive random variables, this is rather trivial.

What is the point of this log transformation for braintics original question though?
 
Status
Not open for further replies.

Users Who Are Viewing This Thread (Users: 0, Guests: 4)

Top