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D2 6th June 2013

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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?

I did it a couple of weeks ago and it seemed fine, messed up on simplex but that was because of me not double checking the calculation :tongue:

what was wrong with the dynamic programming question? :colondollar:

Mark scheme for question 5 seems wrong though now that i look back at it :confused:

Reply 161

mathswiz:)

Original post by Lilmzbest

I did it a couple of weeks ago and it seemed fine, messed up on simplex but that was because of me not double checking the calculation :tongue:

what was wrong with the dynamic programming question? :colondollar:

Mark scheme for question 5 seems wrong though now that i look back at it :confused:

I agree on question 5, there's definitely so dominance going on here :biggrin:

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Reply 162

knowledgecorruptz

Original post by Lilmzbest

I did it a couple of weeks ago and it seemed fine, messed up on simplex but that was because of me not double checking the calculation :tongue:

what was wrong with the dynamic programming question? :colondollar:

Mark scheme for question 5 seems wrong though now that i look back at it :confused:

Seriously? The dynamic programming went right over my head. I think I've got it now but I really hope we can just take it from a graph. Or they start us off.

Admittedly the rest of it was just silly mistakes but the simplex went on for AGES. Usually it's one iteration. And yeah, the mark scheme seems to be answering a different question entirely for 5.

Reply 163

BobLoblawLawBlog

Original post by Lilmzbest

I did it a couple of weeks ago and it seemed fine, messed up on simplex but that was because of me not double checking the calculation :tongue:

what was wrong with the dynamic programming question? :colondollar:

Mark scheme for question 5 seems wrong though now that i look back at it :confused:

Yeah, for question 5 are they answering a completely different question?

Edit: Don't worry, just looked at post above and someone else said the same thing :tongue:

Reply 164

Mallika

Why does converting a network into a complete network of least distances mean that the classical and practical problems are the same?

Reply 165

Hamburglar

Original post by Mallika

Why does converting a network into a complete network of least distances mean that the classical and practical problems are the same?

Classical = Visiting each vertex only once. Practical = Visiting each vertex at least once. If you have a classical problem, then you also have a practical problem.

The reason it gets converted to a classical problem is because the algorithms used visit each vertex only once. Nearest neighbour starts at a vertex, visits every other vertex once only and returns to the start.

However, finding a lower bound by using RMST + 2 smallest arcs doesn't give you a classical problem - that is why they say it is not a viable solution. This is also why when dealing with inequalities, it is strict with respect to the lower bound, but inclusive of the upper bound.

Hope this helps :smile:

Reply 166

Lilmzbest

Original post by mathswiz:)

I agree on question 5, there's definitely so dominance going on here :biggrin:

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Original post by knowledgecorruptz

Original post by brittanna

Yeah, for question 5 are they answering a completely different question?

Edit: Don't worry, just looked at post above and someone else said the same thing :tongue:

Can i ask what everyone got for that? I got p = 4/11 and value = -2/11 ... dunno if it's right though :tongue:

Reply 167

RYRK

Original post by smith50

I was stuck on the same question check post 129 on the previous page someone kindly drew a diagram :smile:

Smith

i still don't get it, if you have the time can you put a explanation to what he has done, thanks

Reply 168

knowledgecorruptz

Original post by Lilmzbest

Can i ask what everyone got for that? I got p = 4/11 and value = -2/11 ... dunno if it's right though :tongue:

Yep, same here.

Reply 169

BobLoblawLawBlog

Original post by Lilmzbest

Can i ask what everyone got for that? I got p = 4/11 and value = -2/11 ... dunno if it's right though :tongue:

Yeah, I got the same as well. It wasn't really a very good paper though as there weren't any proper Hungarian or transportation problem questions.

Reply 170

Lilmzbest

Original post by knowledgecorruptz

Yep, same here.

Original post by brittanna

Yeah, I got the same as well. It wasn't really a very good paper though as there weren't any proper Hungarian or transportation problem questions.

Awesome, lol i've just started ignoring mark schemes lately, they all seem to have mistakes :tongue:

Yeah the practice papers seem to focus on too much formulating linear programming and not enough actual questions :tongue:

Reply 171

smith50

Could someone give me examples of a travelling salesman problem and network flows please :smile:

Thanks,
Smith

Reply 172

BobLoblawLawBlog

Original post by Lilmzbest

Awesome, lol i've just started ignoring mark schemes lately, they all seem to have mistakes :tongue:

Yeah the practice papers seem to focus on too much formulating linear programming and not enough actual questions :tongue:

For dynamic programming, can they ever ask us minimax/ maximin questions from a table, or will they only do that with a network. I really hope we get a network rather than a table in our exam though!

Reply 173

BobLoblawLawBlog

Original post by smith50

Could someone give me examples of a travelling salesman problem and network flows please :smile:

Thanks,
Smith

Do you mean real life examples, or practice questions?

Reply 174

smith50

Original post by brittanna

Do you mean real life examples, or practice questions?

real life problems :smile:

Smith

Reply 175

BobLoblawLawBlog

Original post by smith50

real life problems :smile:

Smith

For network flows, I usually say you can model cars moving through a one way traffic system. Or water flowing though a network of pipes.

For travelling salesman, I guess something like if an inspector wants to travel fro his office to several different restaurants in the shortest time possible and return to wherever his office is based.

Reply 176

Lilmzbest

Original post by brittanna

For dynamic programming, can they ever ask us minimax/ maximin questions from a table, or will they only do that with a network. I really hope we get a network rather than a table in our exam though!

They can ask minimax/maximin from a table? i thought it was always from a network :colondollar:

Reply 177

BobLoblawLawBlog

Original post by Lilmzbest

They can ask minimax/maximin from a table? i thought it was always from a network :colondollar:

I'm not sure. But in the textbook on the last point of the summary box, it says:

You can use dynamic programming to solve minimax and maximin problems presented in network and table form :s-smilie:

Reply 178

Lilmzbest

Original post by brittanna

I'm not sure. But in the textbook on the last point of the summary box, it says:

You can use dynamic programming to solve minimax and maximin problems presented in network and table form :s-smilie: