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Another Impossible D1 Paper? [Edexcel] watch

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    Max flow was 113
    Min cut was a pretty rounabout one
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    (Original post by goku999)
    i did get 109 for the flow but i changed it to 117.
    i found a min cut which was 117 so i assumed it was 117.

    i think i messed the flows question.
    I can't remember my final answer (though it's equal to [initial flow] + 13). When I did that, there were two alternatives for the minimum cut - either using the left-most arcs or the right-most arcs.
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    I got 113 as my flow... but the min cut was difficult to find..
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    ok so i guess my answer of 99 for the flow question is completely wrong!!! altgo its wierd cos i worked out flows going in and those going out and both came to 99???
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    I am definite that the using a backflow the maximum flow is 113, it can be proven by a min.cut.

    I think Djiksra's Algorithm gave 53, as you point out.

    I agree with the profit being 9100 after teh first iteration, however someone mentioned that you pick the least positive value from the objective row to get the pivot, which is wrong - you need the biggest negative (unless it was explicitly specified some other condition, which I should hope did not happen).

    I got two pretty long alternating paths for the matching questions, one of the paths actually embraced all vertex.

    As for the dummies, I used two in the very very last stage of the drawing, so that I could draw some vertex into a sink (i thought that this was a pretty straight-forward sketch, however them mentioning two dummies confused me).

    I believe it was a harder paper than the last nine exams (the papers of which I have done twice. Each.), however this is good - don't spend your summer pittying yourself - if the paper was hard then the grade boundaries are gonna be lower.

    Good luck.
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    my alternating paths were almost identical to each other A and D swapped their matching i think? yeah, i suppose we shouldnt worry that much. i get to look forward to C2 and M2 now, hoorah!
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    a-1 and d-3 i think
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    (Original post by Kiril)
    I am definite that the using a backflow the maximum flow is 113, it can be proven by a min.cut.

    I think Djiksra's Algorithm gave 53, as you point out.

    I agree with the profit being 9100 after teh first iteration, however someone mentioned that you pick the least positive value from the objective row to get the pivot, which is wrong - you need the biggest negative (unless it was explicitly specified some other condition, which I should hope did not happen).

    I got two pretty long alternating paths for the matching questions, one of the paths actually embraced all vertex.

    As for the dummies, I used two in the very very last stage of the drawing, so that I could draw some vertex into a sink (i thought that this was a pretty straight-forward sketch, however them mentioning two dummies confused me).

    I believe it was a harder paper than the last nine exams (the papers of which I have done twice. Each.), however this is good - don't spend your summer pittying yourself - if the paper was hard then the grade boundaries are gonna be lower.

    Good luck.

    You pick the pivotal column then pick the least positive theta value to find the pivotal element
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    (Original post by Kiril)
    I agree with the profit being 9100 after teh first iteration, however someone mentioned that you pick the least positive value from the objective row to get the pivot, which is wrong - you need the biggest negative (unless it was explicitly specified some other condition, which I should hope did not happen).
    It was me who said that, and I was correct. You do not consider negative theta values. You only consider most negative entry in the objective row, when choosing the pivotal column.

    If you cast your eyes to page 169 of the (new) D1 book, paragraph 2:

    (Original post by Heinemann D1 book)
    Only theta-values for rows in which the entries in the pivotal column are positive are used.
    positive is actually in italics in the book as well
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    Does anybody have a copy of the paper and the worked solutions?

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    (Original post by Hash)
    It was me who said that, and I was correct. You do not consider negative theta values. You only consider most negative entry in the objective row, when choosing the pivotal column.

    If you cast your eyes to page 169 of the (new) D1 book, paragraph 2:



    positive is actually in italics in the book as well
    It sounds like you're both correct but getting mixed up between the objective row and theta values.
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    I completely messed up this exam! I did the entire simplex method instead of just step 1 and then ran outta time to complete the paper, absolutely gutted!
    Hopefully my mark from jan and the exam today will drag me grades to a B or hopefully A but unfortunately its doubtful.
    Is it 80% overall to get an A?
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    I was 99% positive that the max flow was 109. As all the vertices leading from the sink were at max capacity. Guess i was wrong.

    Got 9100 for the profit. And showed the next pivotal solution to be of the lowest possible magnitude, which i think was the positive value anyways.

    Couldn't get the graph planar for the life of me.

    Apart from that i thought a very nice paper. A lot easier than any other paper i have ever done and the simplex method question was lovely. Except if any of you made more iterations, it got very nasty :P

    From someone who took the paper last year, id think they would increase the marks needed, however am unsure. I am the only one who got a D last year. 15 people in my class got 100% and all others got A's so im unsure how it will all pan out.
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    The dummies was a hard bit, i was pretty sure it could be done using one dummy.

    53 for Djkstra's Algorithm and 54 for next bit.

    Paths was easy as well.
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    Model answers for D1 paper out there anywhere?
 
 
 
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