• Best of luck to the class of 2024 for their HSC exams. You got this!
    Let us know your thoughts on the HSC exams here
  • YOU can help the next generation of students in the community!
    Share your trial papers and notes on our Notes & Resources page
MedVision ad

Official BOS Trial 2016 Thread (2 Viewers)

RCbest

New Member
Joined
Sep 4, 2014
Messages
4
Gender
Male
HSC
2016
Yeeeee boiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
 

Drsoccerball

Well-Known Member
Joined
May 28, 2014
Messages
3,650
Gender
Undisclosed
HSC
2015
For those who can't do the question here is what i did (may be wrong) :

a) First we choose k numbers out of the n and the out of the remaining n - k numbers we chose m - k of them since total selected is m.



b) Consider all possible combinations. Firstly if bob selects 0 cards, then when bob selects 1 card, , and so on up to when he picks all m.








[]
 
Last edited:

Carrotsticks

Retired
Joined
Jun 29, 2009
Messages
9,494
Gender
Undisclosed
HSC
N/A
For those who can't do the question here is what i did (may be wrong) :

a) First we choose k numbers out of the n and the out of the remaining n - k numbers we chose m - k of them since total selected is m.



b) Consider all possible combinations. Firstly if bob selects 0 cards, then when bob selects 1 card, , and so on up to when he picks all m.






EDIT: So first we pick the m cards and then decide whether or not to put them in one of two places.
[]
Mostly correct, except the last line should have the 2's multiplied instead of added. Well done.
 

leehuan

Well-Known Member
Joined
May 31, 2014
Messages
5,805
Gender
Male
HSC
2015

EDIT: So first we pick the m cards and then decide whether or not to put them in one of two places.
[]
Ahh that was basically what I was thinking :)

I just wasn't so sure about it, especially how to word it
 

Yunsar

New Member
Joined
Jun 22, 2015
Messages
3
Gender
Undisclosed
HSC
N/A
Alternative method for part b:

 
Last edited:

Carrotsticks

Retired
Joined
Jun 29, 2009
Messages
9,494
Gender
Undisclosed
HSC
N/A
A bit heavy for two marks though.
The two marks was aimed at the combinatoric proof. If students could not see how to prove it combinatorically, I gave them the option of proving it from scratch algebraically (as you did). A bit more work than for 2 marks, yes. But better than receiving no marks because they are forced to use only one technique.
 

Trebla

Administrator
Administrator
Joined
Feb 16, 2005
Messages
8,375
Gender
Male
HSC
2006
A bit heavy for two marks though.
Not really if you argue how the result holds in terms of the arrangements in the question.

Even then, algebraically you just need to show is that

 

Yunsar

New Member
Joined
Jun 22, 2015
Messages
3
Gender
Undisclosed
HSC
N/A
Not really if you argue how the result holds in terms of the arrangements in the question.

Even then, algebraically you just need to show is that

Nice work. Certainly quicker than the approach I showed previously.
 

Users Who Are Viewing This Thread (Users: 0, Guests: 2)

Top