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# Edexcel A2 D2 Mathematics June 2016 - Official Thread watch

1. What's going on in the mark scheme for this ?
question? https://3cb8366df74746bcd8eec8f4a703...%20Edexcel.pdf

I don't know what's happening in the first table, I just wrote the second table, then there was no row reduction. I also had 25 instead of the 55, this doesn't matter right?
2. Can anyone explain to me what this means?

The labelling procedure has been used and the result drawn on Diagram 1 in the answer book. (c) Use Diagram 1 to find the maximum flow through the network. You must list eachflow-augmenting route you use, together with its flow.(4) (d) Draw a maximum flow pattern on Diagram 2 in your answer book.
3. When finding an integer solution from a simplex tableau, what happens if a value of one of the slack variables is not 0. e.g. attempting to find an integer solution for example 11 chapter 4 in the d2 textbook. Presumably this just alters one of the constraints and the rest is as normal (so really it is not that different?)?
4. (Original post by AlmostNotable)
We had the whole year to learn it, what's wrong with us?
Duno lol.

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5. (Original post by NotNotBatman)

What's going on in the mark scheme for this ?
question? https://3cb8366df74746bcd8eec8f4a703...%20Edexcel.pdf

I don't know what's happening in the first table, I just wrote the second table, then there was no row reduction. I also had 25 instead of the 55, this doesn't matter right?
As the problem is a maximizing problem, you are first required to subtract all the numbers away from a number which is greater than or equal to the largest one there. Then do the row/column reductions.

The number 60 was picked randomly as it is a bigger number than 27. The idea of picking a value significantly larger than the others is so that it will never get picked in the final answer.
6. (Original post by NotNotBatman)

What's going on in the mark scheme for this ?
question? https://3cb8366df74746bcd8eec8f4a703...%20Edexcel.pdf

I don't know what's happening in the first table, I just wrote the second table, then there was no row reduction. I also had 25 instead of the 55, this doesn't matter right?
The MS looks right. Why wouldnt there be row reduction?
7. Can someone explain to me how to do 3c,d,e on June 2010

https://3cb8366df74746bcd8eec8f4a703...%20Edexcel.pdf
8. (Original post by fpmaniac)
Can someone explain to me how to do 3c,d,e on June 2010

https://3cb8366df74746bcd8eec8f4a703...%20Edexcel.pdf
june 2010 or 13?
9. (Original post by fpmaniac)
The MS looks right. Why wouldnt there be row reduction?
(Original post by Repinski)
As the problem is a maximizing problem, you are first required to subtract all the numbers away from a number which is greater than or equal to the largest one there. Then do the row/column reductions.

The number 60 was picked randomly as it is a bigger number than 27. The idea of picking a value significantly larger than the others is so that it will never get picked in the final answer.

I've realised what I've done wrong, I subtracted from the largest number in each row, somehow got the solution though.
The first table shows that all numbers were subtracted from 30, but why?
Why not just subtract all numbers from 27 and then put some large number in for the "unattractive" cell.
10. (Original post by KloppOClock)
june 2010 or 13?
Jun 13 I meant

-For 3c the Model answers show 3 different routes and the MS shows 3 routes then 4 routes below it.
- For 3d Did the just add the forward flow and the backward flow.
- 3e.. Why have they drawn the minimum cut like that
11. (Original post by fpmaniac)
Jun 13 I meant

-For 3c the Model answers show 3 different routes and the MS shows 3 routes then 4 routes below it.
- For 3d Did the just add the forward flow and the backward flow.
- 3e.. Why have they drawn the minimum cut like that

for c and d there could be different answer which is why the mark schemes say e.g.

as long as you have the correct max flow, and every arc has the same flow in and out and nothing exceeds the capacity of any arcs then you will have it corrects.

for a minimum cut, the cut has to completely seperate the source from the sink and can only pass through:
saturated arcs going from the left to the right
empty arcs going from the right to the left
12. (Original post by KloppOClock)
for c and d there could be different answer which is why the mark schemes say e.g.

as long as you have the correct max flow, and every arc has the same flow in and out and nothing exceeds the capacity of any arcs then you will have it corrects.

for a minimum cut, the cut has to completely seperate the source from the sink and can only pass through:
saturated arcs going from the left to the right
empty arcs going from the right to the left
So it can basically be anywhere as long as it passes through saturated arcs and divides the diagram roughly in half because the MS and Model answers show different cuts
13. (Original post by fpmaniac)
So it can basically be anywhere as long as it passes through saturated arcs and divides the diagram roughly in half because the MS and Model answers show different cuts
not just saturated arcs, empty arcs as well going the other direction.

but yes

also it doesnt have to be in half, as long as the cut completely separates the source from the sink
14. (Original post by NotNotBatman)
I've realised what I've done wrong, I subtracted from the largest number in each row, somehow got the solution though.
The first table shows that all numbers were subtracted from 30, but why?
Why not just subtract all numbers from 27 and then put some large number in for the "unattractive" cell.
30 was just the number they picked. I would also pick 27 like you did, but any value is correct so long as it is 27 or more!
15. Has anyone done the jun 14 paper? I don't understand the answer to q7! The Mark scheme has May and April starting with 2 planes in stock even though that's impossible to do because you can only make 4 in one month and you can only store 2? I got the right solution but lost loads of marks for not including impossible rows?

So say in March you can either make 3 or 4 and carry forward a maximum of 1 not 2.
In April you may start with one and make 3 or 4 and carry only 1 forward.

I don't see how you can start May and April with 2 in stock?

*NEVERMIND I LOOKED AT THE EXAMINER REPORT*

But could anyone explain for the network flows how they've sent flow down BA and DE when the arrow doesn't point that way?
*NEVERMIND I FORGOT THAT BACK FLOW EXISTED*
Attached Images
16. Question-paper-Unit-D2-(6690)-June-2014.pdf (180.9 KB, 69 views)
17. Mark-scheme-Unit-D2-(6690)-June-2014.pdf (200.8 KB, 49 views)
18. (Original post by KloppOClock)
not just saturated arcs, empty arcs as well going the other direction.

but yes

also it doesnt have to be in half, as long as the cut completely separates the source from the sink
Sorry for all the rookie questions, but how did they find the empty arc without going through augmenting process. In the book they have augmented one route then another route and then carried on until the Vertices after the first one all have saturated arcs.

19. In the mark scheme it says one flow augmenting route is SBADHT, but BA has max flow in the arc, I think this has something to do with backflows, but I can't remember how they work; could someone explain please?
20. Good luck all!!

Need to learn DP still - the worded ones are horrendous. I also mess up a bit on network flows but I can attempt them at least. DP grind for the win! Gonna watch some videos on them and go on the question by topic section and try my best. I've hated a lot of my exams this year, let's hope this isn't one of them!!
21. (Original post by NotNotBatman)

In the mark scheme it says one flow augmenting route is SBADHT, but BA has max flow in the arc, I think this has something to do with backflows, but I can't remember how they work; could someone explain please?
That's confused me. Surely that means that the spare capacity is zero so therefore there cannot be a route along that path as BA has no possibility of change >0? Is this a mistake in the markscheme?

Forward arrow = spare capacity.
Backward Arrow = how much the flow can be reduced by.
It's saturated so therefore I cannot see how there can be a route.
22. (Original post by TrueDAN)
That's confused me. Surely that means that the spare capacity is zero so therefore there cannot be a route along that path as BA has no possibility of change >0? Is this a mistake in the markscheme?

Forward arrow = spare capacity.
Backward Arrow = how much the flow can be reduced by.
It's saturated so therefore I cannot see how there can be a route.
Actually I think I understand it now. You're changing the direction of the flow, so that every arc in the route following A would have an extra 2 added to the flow.

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