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# D2 6th June 2013 Watch

1. (Original post by Mallika)
Thank you again, I think I understand :P
Another question if you don't mind

I was doing practice paper B and saw the LP question was unbalanced- this means if you were asked to solve it using simplex you would need to add slack variables right?
If the supply=demand and the question was balanced, would you still use inequalities or use an equals sign? In the textbook they use inequality signs (so in theory you would need to add slack variables if you were to solve it) but wouldn't they all be 0?
I think regardless of whether it is balanced or not, you always use inequalities and you always use slack variables.

I agree, I think they would be 0 but I'm not 100% sure, I don't see why they wouldn't be though.

But you definitely use slack variables because in game theory, you know the probabilities should equal 1. p1 + p2 + p3 = 1. However, you still use an inequality and include a slack variable. This is useful because it is a check variable, and if it comes out to be 0 then you've done it correctly.

I would assume, that is also what is going on here.
2. (Original post by ChelseaSam)
Also, Can anyone tell me if doing row operations like this would be credited all the marks? The answer is correct.
anyone know?
3. (Original post by Hamburglar)
I think regardless of whether it is balanced or not, you always use inequalities and you always use slack variables.

I agree, I think they would be 0 but I'm not 100% sure, I don't see why they wouldn't be though.

But you definitely use slack variables because in game theory, you know the probabilities should equal 1. p1 + p2 + p3 = 1. However, you still use an inequality and include a slack variable. This is useful because it is a check variable, and if it comes out to be 0 then you've done it correctly.

I would assume, that is also what is going on here.
PRSOM
Thank you again That makes sense actually, and that check is going to be a lifesaver in the exam!
4. (Original post by Mallika)
PRSOM
Thank you again That makes sense actually, and that check is going to be a lifesaver in the exam!
Aw, returned

Yeah, although I haven't seen any game theory linear programming or even transportation linear programming come up before in terms of simplex. Usually the simplex stuff is it's own isolated question. But who knows, we could be the first :s
5. Could someone help me on this question please how do we use the capacities to figure out the flow?

Thanks,
Smith
6. Hello there guys. Could somebody please answer a couple of questions that I have on D2?

Firstly, when formulating a game theory linear programming problem, should you leave the constraints as >= or introduce slack variables?

Secondly, when maximising an allocation problem where the data is incomplete, do you treat the incomplete table entries as zeros and then continue as normal, subtracting every value in the table from a number greater than or equal to the largest data entry?

Thanks in advance to whoever can help .
7. (Original post by Hamburglar)
Aw, returned

Yeah, although I haven't seen any game theory linear programming or even transportation linear programming come up before in terms of simplex. Usually the simplex stuff is it's own isolated question. But who knows, we could be the first :s
I bet we will be the unlucky ones I just hope game theory LP doesn't come up , the rules for it are a bit different aren't they? (Usually the theta values should be positive but for GT, you can use 0 e.t.c)
Have you finished all the past papers?
8. (Original post by leo99756)
Hello there guys. Could somebody please answer a couple of questions that I have on D2?

Firstly, when formulating a game theory linear programming problem, should you leave the constraints as >= or introduce slack variables?

Secondly, when maximising an allocation problem where the data is incomplete, do you treat the incomplete table entries as zeros and then continue as normal, subtracting every value in the table from a number greater than or equal to the largest data entry?

Thanks in advance to whoever can help .
Hi,
For the first point it depends on the question it will say so
For the second point first reduce the entries by the largest value then enter a value into the table which is at least twice the largest value in the table.Then reduce rows then columns as normal.Hope this helps
Could you help me with the question i have posted please thanks
Smith
9. (Original post by Mallika)
I bet we will be the unlucky ones I just hope game theory LP doesn't come up , the rules for it are a bit different aren't they? (Usually the theta values should be positive but for GT, you can use 0 e.t.c)
Have you finished all the past papers?
Hi could you please explain the flows question i posted earlier please

Thanks,
Smith
10. (Original post by smith50)
Hi could you please explain the flows question i posted earlier please

Thanks,
Smith
I can't work it out, what values did you get for the cuts? I'm getting 40 for C1 and 66 for C2 which looks wrong
I think you could work it out if you found the min cut because then any arcs going into that cut must be saturated and any arcs going out would be empty, using that you could find out what GT was. But it won't work out for me :/

EDIT: I think I've got it- the maximum cut would be the smaller one (40) because using the larger cut means the conservation conditions isn't satisfied. So AF, DF and DG are saturated, so by the conservation condition again GT must be 24
11. (Original post by Mallika)
I bet we will be the unlucky ones I just hope game theory LP doesn't come up , the rules for it are a bit different aren't they? (Usually the theta values should be positive but for GT, you can use 0 e.t.c)
Have you finished all the past papers?
Yeah that's true

Yep, the past papers and the 2 practice papers, although I haven't bothered with anything older than that, I'm hoping that there aren't any old-style questions on the current spec.

12. (Original post by Hamburglar)
Yeah that's true

Yep, the past papers and the 2 practice papers, although I haven't bothered with anything older than that, I'm hoping that there aren't any old-style questions on the current spec.

I've done no papers yet I'm going through the spec paper, practice A and D and the 2001 paper going by question type, once I'm confident enough I'll start the new spec papers Though that will probably be tomorrow!
13. (Original post by Mallika)
I can't work it out, what values did you get for the cuts? I'm getting 40 for C1 and 66 for C2 which looks wrong
I think you could work it out if you found the min cut because then any arcs going into that cut must be saturated and any arcs going out would be empty, using that you could find out what GT was. But it won't work out for me :/

EDIT: I think I've got it- the maximum cut would be the smaller one (40) because using the larger cut means the conservation conditions isn't satisfied. So AF, DF and DG are saturated, so by the conservation condition again GT must be 24
Thank you could you explain this question to me please
Smith
14. (Original post by Mallika)
I've done no papers yet I'm going through the spec paper, practice A and D and the 2001 paper going by question type, once I'm confident enough I'll start the new spec papers Though that will probably be tomorrow!
That's quite a lot for one day! Good luck Honestly you'll find that the more recent papers are all very straightforward. The practice papers and the older ones are a bit dodgier in my opinion

(Original post by smith50)
Thank you could you explain this question to me please
Smith
I'll take over

I think the flow should be increased along BE. This is because the minimum cut passes through BE, therefore the maximum flow from S through to T partially passes through this arc.

Increasing the capacity of this arc, increases the capacity of the minimum cut, and therefore the maximum flow
15. Anyone have any revision notes for this module? Not much available on the web.
16. (Original post by Hamburglar)
That's quite a lot for one day! Good luck Honestly you'll find that the more recent papers are all very straightforward. The practice papers and the older ones are a bit dodgier in my opinion
I second this, i've been doing the papers in chronological order and they seem to be getting less tricky ... althought that may be due to me gettting more practice i suppose
17. (Original post by Hamburglar)
That's quite a lot for one day! Good luck Honestly you'll find that the more recent papers are all very straightforward. The practice papers and the older ones are a bit dodgier in my opinion

I'll take over

I think the flow should be increased along BE. This is because the minimum cut passes through BE, therefore the maximum flow from S through to T partially passes through this arc.

Increasing the capacity of this arc, increases the capacity of the minimum cut, and therefore the maximum flow
Sorry why BE not any other arc?
Smith
18. Ugh. D2 is awful. I haven't started doing anything for it yet and I don't know how to do anything. The exam may be quite hard for me.
19. Can someone please post all the real life examples for each topic, ie. travelling salesman????????
20. (Original post by smith50)
Thank you could you explain this question to me please
Smith
I haven't actually done the question but looked at the answer-

the question is asking you to look at your final flow (the one you worked out) and work out how you could maximise the flow again ie make it as large as possible.
With CF, if you increased the capacity of the arc you still would not change the flow through it because the arc going into C (BC) is full
With DH, again both arcs going into node D are saturated so if you increases DH no "extra" could flow in anyway
So BE should be increased (the arcs going into B aren't saturated)

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