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# D2 6th June 2013 watch

1. (Original post by smith50)
Oh i'm sorry it's post 110
Smith
Aah okay, are you having trouble with part e? Its just a matter of seeing how much extra flow an arc will allow

Can't see the question that well, what paper is it?
2. (Original post by Lilmzbest)
Aah okay, are you having trouble with part e? Its just a matter of seeing how much extra flow an arc will allow

Can't see the question that well, what paper is it?
It's cool someone has already posted some working i get the question now thanks for your posts it's jan 2006 D1 Q4
Smith
3. (Original post by sacheeen)
Hi guys,

I'm a bit confused when graphing a game theory problem. The optimal point - which one is it?
I've resorted to finding the intersection of all the lines and find the point which gives the highest value but this isn't correct for some reason. Can anyone shed some light on this matter?
Same in some i get it right but not in others it's quiet confusing
Smith
4. (Original post by sacheeen)
Hi guys,

I'm a bit confused when graphing a game theory problem. The optimal point - which one is it?
I've resorted to finding the intersection of all the lines and find the point which gives the highest value but this isn't correct for some reason. Can anyone shed some light on this matter?
When you have all the lines, you need to look along the line that goes across the bottom. You then look for the highest point on this line, and use the two lines that intersect at this point.
5. (Original post by brittanna)
When you have all the lines, you need to look along the line that goes across the bottom. You then look for the highest point on this line, and use the two lines that intersect at this point.
If you don't mind could you illustrate what you mean
Smith
6. (Original post by smith50)
If you don't mind could you illustrate what you mean
Smith
I don't know if this helps, but i've drawn two examples of the graphs you end up drawing. I have drawn along the minimum line in bold and have circled the max point on this line.

If you don't get it, I can try and explain it again
Attached Images

7. (Original post by brittanna)
I don't know if this helps, but i've drawn two examples of the graphs you end up drawing. I have drawn along the minimum line in bold and have circled the max point on this line.

If you don't get it, I can try and explain it again
I think i get it now thanks, you explain better than the textbook itself
Could you explain it aswell please just to reassure myself
Thanks,
Smith
8. (Original post by smith50)
I think i get it now thanks, you explain better than the textbook itself
Could you explain it aswell please just to reassure myself
Thanks,
Smith
(Original post by Arsey)
are you referring to the well graph?

shade in from the bottom of the well until you hit the lines (think the fill option in MS Paint) - the highest point shaded represents the best strategy for the player whose pay-off matrix you have been solving.

Solve the simultenous equations on the equations of the lines which intersect, this will give you p, sub back into either of the equations to find the value of the game.
Arsey has also explained it very well here.

So on the diagrams I had, you shade everything in until you hit a line (although I don't think the x axis line counts), and then the highest point is the optimal one.
9. (Original post by brittanna)
Arsey has also explained it very well here.

So on the diagrams I had, you shade everything in until you hit a line (although I don't think the x axis line counts), and then the highest point is the optimal one.
Thanks once again i understand it now
Smith
10. Hi, I keep seeing "give an example of a practical situation that can be modelled by a network flow" or "suggest a practical situation where a maximin / minimax route can be used?". All the MS says is "an idea of a directed flow from s to t". Do I put this as my answer or do I say something like... the Electric grid sending out electricity...
11. quick question ,when doing the travelling sales man problem , do you have find the least distances first , then do kruskals or prims algorithm?
12. (Original post by ChelseaSam)
Hi, I keep seeing "give an example of a practical situation that can be modelled by a network flow" or "suggest a practical situation where a maximin / minimax route can be used?". All the MS says is "an idea of a directed flow from s to t". Do I put this as my answer or do I say something like... the Electric grid sending out electricity...
For a typical network flow, I imagine electricity grid wouldn't be a bad thing to say. Or anything like piping water to homes, or things that involve distribution.

For minimax and maximin the book outlines two examples (which have also come up on a past paper).

Minimax: Minimise the greatest distance a plane has to fly between airports so that fuel can be minimised. This then allows you to maximise the cargo that it can carry. (This one is a bit weird, but consider that the plane can refuel at each airport, and also consider that the more fuel it needs, the less physical space there is for cargo).
Maximin: Maximise the slowest process in a factory which allows you to increase the overall rate of goods produced. (Think of rate-determining step if you do chemistry).

Hope this helps.

(Original post by RYRK)
quick question ,when doing the travelling sales man problem , do you have find the least distances first , then do kruskals or prims algorithm?
If they want you to they will almost certainly ask you to find a table of least distances first, so yes. Otherwise, you can do Kruskal's or Prim's directly from the graph given using techniques from D1. You could find your own table of least distances if you wanted to but it would be way more inefficient, and I suggest you just use the graph.
13. Holy ****, has anyone tried the D2 Practice Paper B?

I mean, what on earth!? The LP question is unbalanced. so you have to turn everything into inequalities?

And then the dynamic programming question is impossible? Honestly, did anyone here get that one correct?
14. (Original post by knowledgecorruptz)
Holy ****, has anyone tried the D2 Practice Paper B?

I mean, what on earth!? The LP question is unbalanced. so you have to turn everything into inequalities?

And then the dynamic programming question is impossible? Honestly, did anyone here get that one correct?
Doing it today/tomorrow, but yesterday I was posting about that unbalanced LP.

You just treat it as any other LP problem apparently. You don't have to add dummies or anything, you just have to take each row and say it's less than the supply, and do the same with the columns and demand just as in any other LP problem for transportation.
15. (Original post by Hamburglar)
Doing it today/tomorrow, but yesterday I was posting about that unbalanced LP.

You just treat it as any other LP problem apparently. You don't have to add dummies or anything, you just have to take each row and say it's less than the supply, and do the same with the columns and demand just as in any other LP problem for transportation.
Yeah, I remember but I didn't think you'd have to do it for the columns as the number supplied should still equal the demand. Idk, I'll just do the inequalities thing - even if it doesn't make total sense to me.

Try the papers and let me know how you find them. I think they're ridiculous compared to the actual exam papers.
16. (Original post by knowledgecorruptz)
Yeah, I remember but I didn't think you'd have to do it for the columns as the number supplied should still equal the demand. Idk, I'll just do the inequalities thing - even if it doesn't make total sense to me.

Try the papers and let me know how you find them. I think they're ridiculous compared to the actual exam papers.
Yeah the thing is, you've given the rows as inequalities, which means you have given the amount of supply sent out as inequalities. Therefore the columns (demand) must also be specified as inequalities. We can't just say supply = demand because we've given supply as an inequality, and not as an equation.

If the problem is unbalanced like in this case, then yeah it is a bit weird. You give the supply as inequalities and the demand as inequalities - and if the supply sent out is greater than the overall demand then it will no longer satisfy the demand inequality which is weird, but I think that's just how you're supposed to do it :s

I'll have a go I've already done practice paper A - I thought it was okay to be honest. The transportation problem question was a bit different but there wasn't anything too shocking in the paper, although I forgot to reduce columns in the hungarian algorithm, AGAIN
17. can some one tell me how to do question 7 on the adapted 2006 jan D2 paper. heres the link to the paper http://www.thestudentroom.co.uk/atta...2&d=1338067181
18. (Original post by RYRK)
can some one tell me how to do question 7 on the adapted 2006 jan D2 paper. heres the link to the paper http://www.thestudentroom.co.uk/atta...2&d=1338067181
I was stuck on the same question check post 129 on the previous page someone kindly drew a diagram
Smith
19. (Original post by Hamburglar)
Yeah the thing is, you've given the rows as inequalities, which means you have given the amount of supply sent out as inequalities. Therefore the columns (demand) must also be specified as inequalities. We can't just say supply = demand because we've given supply as an inequality, and not as an equation.

If the problem is unbalanced like in this case, then yeah it is a bit weird. You give the supply as inequalities and the demand as inequalities - and if the supply sent out is greater than the overall demand then it will no longer satisfy the demand inequality which is weird, but I think that's just how you're supposed to do it :s

I'll have a go I've already done practice paper A - I thought it was okay to be honest. The transportation problem question was a bit different but there wasn't anything too shocking in the paper, although I forgot to reduce columns in the hungarian algorithm, AGAIN
Haven't done A, will try it soon - try B. Ah yes, that makes sense (sort of) now, thank you

Haha, never had that problem. It's always simplex that catches me out - it's too damn fiddly!

Also: with the hungarian algorithm, should the number of lines used increase every time (definitely)?
20. (Original post by knowledgecorruptz)
Haven't done A, will try it soon - try B. Ah yes, that makes sense (sort of) now, thank you

Haha, never had that problem. It's always simplex that catches me out - it's too damn fiddly!

Also: with the hungarian algorithm, should the number of lines used increase every time (definitely)?
Alright Let me know how A goes.

Yeah it's so easy to make an arithmetic error on simplex, especially with so many fractions, sometimes it is just impossible to avoid.

And no I've noticed that sometimes the number of lines stays the same, but it should never decrease obviously. If I remember correctly, on practice paper A I had the same number of lines on the first two iterations - it does depend on how you draw your lines though.

EDIT: Like I said though I messed that question up a bit because I didn't reduce columns. But I'm pretty sure I've had some where the number of lines has stayed the same :s If anyone else could confirm that'd be great

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