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D2 OCR Network Flows, please help, exam wednesday! watch

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    Hey guys, I seem to always get the questions like "find the value of this cut" wrong on the past papers, and although they're only worth a couple of marks, I'd like to sort it out. After turning to the OCR "Discrete Mathematics 2" textbook, I'm somewhat confused...
    Take example 2.2.1:
    "a.) The value of the cut is 4+1+6=11"... Ok, so far so good. They've added the capacities of the arcs in the direction of S-T, ignoring the one going backwards.
    However, in Exercise 2A: question 1. The values of C2 and C4 are 8+12=20 and 9+15=24, respectively. Now this is fine once again as CA with value 5 was ignored due to its direction, but they seem also to have ignored the arcs DA (for C2) and CB (for C4).
    Now in the examples, they take into account these "vertical" arcs, but in the exercises, they are to be ignored...

    So I guess my question is, if a cut crosses a "vertical" arc, what do I do with that arc's capacity - add it, or ignore it?

    Are they in the direction S-T or T-S?

    thanks.
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    mehh no replies exam tomorrow... apologies for the double post
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    (Original post by korobeiniki)
    Hey guys, I seem to always get the questions like "find the value of this cut" wrong on the past papers, and although they're only worth a couple of marks, I'd like to sort it out. After turning to the OCR "Discrete Mathematics 2" textbook, I'm somewhat confused...
    Take example 2.2.1:
    "a.) The value of the cut is 4+1+6=11"... Ok, so far so good. They've added the capacities of the arcs in the direction of S-T, ignoring the one going backwards.
    However, in Exercise 2A: question 1. The values of C2 and C4 are 8+12=20 and 9+15=24, respectively. Now this is fine once again as CA with value 5 was ignored due to its direction, but they seem also to have ignored the arcs DA (for C2) and CB (for C4).
    Now in the examples, they take into account these "vertical" arcs, but in the exercises, they are to be ignored...

    So I guess my question is, if a cut crosses a "vertical" arc, what do I do with that arc's capacity - add it, or ignore it?

    Are they in the direction S-T or T-S?

    thanks.
    how did the exam go for you then?? what did u get for the dynamic programming question
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    I got 7 tonnes then found a route of 8 by inspection and redid the question, good thing I spotted that I think, because I didn't get a chance to answer the last two parts of q6 properly. I finally got the hang of that ^^^ problem to any interested :P it came to me the night before the exam
 
 
 
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