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# D2 6th June 2013 Watch

1. (Original post by JayJay95)
But the do require that demand. Practically, many companies promise to deliver a certain amount of goods however, unforseen issues occur. Take agriculture as an example. An unforseen drought could occur and destroy half the crop leading to a reduction in supply etc.
Yes, but that has no effect on the solution that I developed theoretically and independently beforehand?
In that question I assume they've introduced slack varibles to absorb the excess supply hence the equalities
No -- they allowed equalities or inequalities for the demand totals (i.e. columns), but they only allowed inequalities for the supply totals (i.e. rows). This reflects what I originally said.
2. (Original post by Miken Moose)
Yes, but that has no effect on the solution that I developed theoretically and independently beforehand?
No -- they allowed equalities or inequalities for the demand totals (i.e. columns), but they only allowed inequalities for the supply totals (i.e. rows). This reflects what I originally said.
I checked that paper and mark scheme but it looks like someone wrote that mark scheme themselves where it says you could use equalities. I remember doing that question and I managed to find it on practice paper b

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3. Okay... I've just noticed something glaringly obvious which should settle this. First, this is the solution laid out by example 12 page 27:

Let be the number of units transported from to .

Minimise

Subject to:

Okay? So, why don't I just set all the ? This satisfies all constraints and clearly gives the minimum value of C.

Unless of course, I'm missing something (which for me is quite likely).
4. You can do that but thats not the only solution. What you've suggested is the practical solution. Like I said since its a linear programming problem theres a range of values

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5. what are your predictions for this paper?
6. (Original post by JayJay95)
You can do that but thats not the only solution. What you've suggested is the practical solution. Like I said since its a linear programming problem theres a range of values
But it isn't, that's precisely the point. I haven't solved the transportation problem at all, I've just said "Ah to hell with this let's not bother" i.e. don't send anything anywhere. The transportation problem is about selecting how many of which supply points to send to which demand points, whilst still satisfying the demand, to minimise transportation costs. Having those constraints as equations still gives you a feasible region of solutions.
7. (Original post by Miken Moose)
But it isn't, that's precisely the point. I haven't solved the transportation problem at all, I've just said "Ah to hell with this let's not bother" i.e. don't send anything anywhere. The transportation problem is about selecting how many of which supply points to send to which demand points, whilst still satisfying the demand, to minimise transportation costs. Having those constraints as equations still gives you a feasible region of solutions.
An equation has only one solution. When you use an equal sign instead of an inequality, you dont have a region if solutions, just one. With an inequality you have multiple solutions thats the whole point if linear programming

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8. The sample assessment material mark scheme uses equality...

But I'm going to go with the textbook as everyone learns it from this text book, so most people will use inequalities
9. (Original post by JayJay95)
An equation has only one solution. When you use an equal sign instead of an inequality, you dont have a region if solutions, just one. With an inequality you have multiple solutions thats the whole point if linear programming

Er... I agree, "an equation [in one variable] has only one solution". But here there are several equations in several variables. There is more than one solution.

But that's not even the point, anyway. By using inequalities you are ignoring what the transportation problem actually is. The solution where all the decision variables are zero ought to fall out of the feasible region because it doesn't solve the problem.

Perhaps I should draw a simple analogy: say I need to buy several items, A,B,C... and so on, but I want to spend as little money as possible. So I could look at several different shops and what have you and then figure out how to get these items most cheaply from there. What you're saying is that a feasible solution is "don't buy anything" -- well, it's not, because I need those items.

Now, you're probably going to say, "but that's a practical example" -- but it doesn't matter. The point is, you're offering a 'solution' to the wrong problem. The issue of minimising costs in any context can be trivialised by not doing it.

Additionally, the image you posted, which I assume is the practice paper -- I would say that this is more likely to be incorrect than a MS for a genuine D2 paper.
10. (Original post by Miken Moose)

Er... I agree, "an equation [in one variable] has only one solution". But here there are several equations in several variables. There is more than one solution.

But that's not even the point, anyway. By using inequalities you are ignoring what the transportation problem actually is. The solution where all the decision variables are zero ought to fall out of the feasible region because it doesn't solve the problem.

Perhaps I should draw a simple analogy: say I need to buy several items, A,B,C... and so on, but I want to spend as little money as possible. So I could look at several different shops and what have you and then figure out how to get these items most cheaply from there. What you're saying is that a feasible solution is "don't buy anything" -- well, it's not, because I need those items.

Now, you're probably going to say, "but that's a practical example" -- but it doesn't matter. The point is, you're offering a 'solution' to the wrong problem. The issue of minimising costs in any context can be trivialised by not doing it.

Additionally, the image you posted, which I assume is the practice paper -- I would say that this is more likely to be incorrect than a MS for a genuine D2 paper.
Ahh now i see what you're getting at. You may need thise items but it doesnt mean you're gonna get em since all the stores in your vicinty dont stock them.

Secondly you're trying to find a solution that minimises the cost when theres no need to since they dont ask you for that. All they do is give you a problem, some stock and demand and ask you to formulate it as a LP. They dont say demand meets supply or anything of that kind. They dont say ANYTHING.

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11. When you have to formulate a maximise Hungarian algorithm as a linear programming problem, why can't you just have the objective function say maximise P=... rather than changing the matrix and doing it as a minimising problem?
12. (Original post by brittanna)
When you have to formulate a maximise Hungarian algorithm as a linear programming problem, why can't you just have the objective function say maximise P=... rather than changing the matrix and doing it as a minimising problem?
because when you use the Hungarian algorithm to decrease the numbers, you will, in turn, reduce the smaller numbers and end up minimizing it and not getting your maximum results i believe its a little confusing with a question but i believe that is correct
13. In travelling salesman.. If the network is not complete... as in the n vertices are not connected to every other vertex - do we have to create a table of least distances in order to use the NN algorithm and prims etc etc or do we use the network we are given? Thanks!
14. (Original post by ChelseaSam)
In travelling salesman.. If the network is not complete... as in the n vertices are not connected to every other vertex - do we have to create a table of least distances in order to use the NN algorithm and prims etc etc or do we use the network we are given? Thanks!
I get the feeling that if we needed to make a table then we would be specifically instructed to do so. They should make it clear as to what we need to do.
15. Why do we always work backwards from dynamic programming?
16. (Original post by JayJay95)
Ahh now i see what you're getting at. You may need thise items but it doesnt mean you're gonna get em since all the stores in your vicinty dont stock them.

Secondly you're trying to find a solution that minimises the cost when theres no need to since they dont ask you for that. All they do is give you a problem, some stock and demand and ask you to formulate it as a LP. They dont say demand meets supply or anything of that kind. They dont say ANYTHING.

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This website uses inequalities: http://my.safaribooksonline.com/book...ch1sec21_xhtml. I think I'm going to stick to using them.
17. (Original post by brittanna)
I meant because i'll have wasted all of the ones they give you from making mistakes .
Your offers conditional of AFM . That's a bit harsh, but yeah, they aren't going to not give you a place if you don't get an A in D2, especially if you're getting S's in STEP .

I need 86 in either D2 or FP2 (I got 94 in S2 Jan), but I think i'm more likely to meet it in FP2.
Oh right, well I'm sure you'll be fine! They do include a stupid amount of tables (at least double what it can be done with!)

I know right! I think it's just to make sure I didn't drop it (and not learn the material) so that I didn't have a year without begin forced to learn new material. I don't think they actually want me to get the grade in it though considering I got my A*A* in the other maths A-levels in year 12. Imperial's offer was quite funny! It arrived in like february when I had already got offers from my top two choices and it was A*A*A*A I can remember reading these horror stories in the national papers a year or two ago about a kid who had got an A*A*A* offer and I thought that was mental. And I also read an article about a kid with AAAAA who got rejected by Cambridge but now I realise all this stuff in the paper that they claimed was strange s just normal! There were two kids at my school over the last two years who didn't get in and one got 5A*s and another got 7

Hoping for S's... I have an unfortunate habit of doing worse on the day than I do in mocks though

Fuuck do you think FP2 is easier? Is that needed for Warwick then? (guessed from the fact you're doing STEP I and AEA)

Just did June 2011 btw! Got 70. I remembered "chose the least positive value" incorrectly for the simplex algorithm and picked the largest
18. (Original post by NiceToMeetYou)
Why do we always work backwards from dynamic programming?
because if you started at the front you would have no 'value' to add onto, if that makes sense. So if it is from January-April if you start in January you have nothing to add onto but as in most past papers i have seen they start you off you can work back using the info given. I believe that is the reason
19. (Original post by Jkn)
Oh right, well I'm sure you'll be fine! They do include a stupid amount of tables (at least double what it can be done with!)

I know right! I think it's just to make sure I didn't drop it (and not learn the material) so that I didn't have a year without begin forced to learn new material. I don't think they actually want me to get the grade in it though considering I got my A*A* in the other maths A-levels in year 12. Imperial's offer was quite funny! It arrived in like february when I had already got offers from my top two choices and it was A*A*A*A I can remember reading these horror stories in the national papers a year or two ago about a kid who had got an A*A*A* offer and I thought that was mental. And I also read an article about a kid with AAAAA who got rejected by Cambridge but now I realise all this stuff in the paper that they claimed was strange s just normal! There were two kids at my school over the last two years who didn't get in and one got 5A*s and another got 7

Hoping for S's... I have an unfortunate habit of doing worse on the day than I do in mocks though

Fuuck do you think FP2 is easier? Is that needed for Warwick then? (guessed from the fact you're doing STEP I and AEA)

Just did June 2011 btw! Got 70. I remembered "chose the least positive value" incorrectly for the simplex algorithm and picked the largest
I didn't even get an Imperial offer (or a Cambridge one, but after my interview i'm really not surprised), and when I asked them why, they said 'we take a wide range of factors into consideration when making offers, please check our entry requirements page for more information' I really wasn't very impressed . I'm hoping for a 1 in STEP I, but I won't be disappointed if I only get a 2. I want a distinction in AEA, but if a vectors question comes up (I have no idea how are you supposed to see half of the things they expect you to see!), then I won't be expecting much from it .

I wouldn't say I think FP2 is easier, it's just that I tend do worse on the applied modules than on the pure modules, and so i'm probably more likely to lose fewer marks on FP2. And yeah, that's for Warwick.

I'm not trying to show of or anything, but I just did practice paper A and got 75 . Although yesterday I did june 2010 and got about 65 (this was mainly due to looking at the wrong tables when doing the transportation one ).

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Updated: June 11, 2013
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