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Decision Maths 1 (thursday) watch

1. As i'm sure aload of you online now are taking the Edexcel D1 exam tomorrow, in this thread please discuss any problems...
This will help me

To start things off...

does anyone find it weird that the exam only gives you half a page to do an entire bubble sort??

Also, In the exam, with linear programming, do i have to draw an 'interim' table, without changing the variables, before forming a new tableau??

Cheers for any help.
2. One more problem for you....

how would i go about proving a graph is non planar??
(obviously i could use the planarity algorithm for a planar graph, but i dont really know what to write for a non-planar - as i could just end up proving that *I* cant draw it as planar)

for people who have an exam tomorrow, you arent exactly flooding in with replies and answers guys.
3. If you want to prove that a graph is not planar, we were always told to see if you could draw it without any lines crossing. So you keep one line inside, and then draw any lines that cross it outside. If it is not possible to draw these lines outside without them crossing, it is not planar.

Sorry this isn't very clear, but its all I have left of my notes from January. Afraid I didn't do edexcel, did ocr instead, so I can't answer any of the other questions because there may be differences in the syllabus. However, I'm happy to help if I can on anything else!

Joanne.
4. thanks,
So i presume that you just draw part of the graph as planar, and explain that the extra line or so cannot be added because they will cross....

can anyone help me out with the linear programming problem, id like to know how you lot do it.(my teacher has taught us not to put the extra table in)
5. Ive been entered for the exam but Im not gonna turn up.
6. yeh my mate is doing that too.
Because we do it as an extra subject we only get 40 minutes a week to learn the course so none of us are planning for a decent grade (or any grade at all really)
7. im doing it tomorrow, the only thing i cant do is the Simplex method, we were never taught it and i cant understand how to do it from the book! Hopefully it wont come up tomorrow!
8. can someone plz explain 2 me wtf is goin on at the top of p170 it makes NO sense wotsoeva??!??????
9. (Original post by DaKe)
can someone plz explain 2 me wtf is goin on at the top of p170 it makes NO sense wotsoeva??!??????
Once you have found the pivot, all the other numbers in the same column must become zero.
This is achieved by taking multiples of the pivotal row (either adding or subtracting) to the other rows.

In the book the pivotal row is row t which is Row 2.
As you can see the row above, row s has a number three in the same column as the pivot.
It has to become zero.
So all u do is do row s - 3 x row t (three times row t)
3 - (3x1)=0

U do this to each variable on row s

so 3 - 3(1/2) = 3/2 (column x)
1 - 3(0) = 0 (column s)
0 - 3(1/4) = -3/4 (column t)
120 - 3(37½) = 7½ (value column)

See tableau 2 page 170.

hope this helps.
10. (Original post by dnbhead)
im doing it tomorrow, the only thing i cant do is the Simplex method, we were never taught it and i cant understand how to do it from the book! Hopefully it wont come up tomorrow!
the simplex algorithm is likely to come up.
Or else there's gonna be a big flows question.
Simpelx algorith is pretty easily to understand as theres a pattern to it.
11. Has anyone got any tips when drawing networks from precedance tables, which include dummies. I get myself so confused!
Also capacity of cuts...... are they just the total capacity of the arcs despite the direction of the flow or do i use only the capacities which the flow goes from source to sink?
Any help much appreciated!
12. (Original post by Toffee_Girl)
Has anyone got any tips when drawing networks from precedance tables, which include dummies. I get myself so confused!
Do it in pencil first so that you can always alter it later. Work your way through the table one stage at a time.

Also capacity of cuts...... are they just the total capacity of the arcs despite the direction of the flow or do i use only the capacities which the flow goes from source to sink?
Any help much appreciated!
Only the capacities going from source to sink.
13. (Original post by Gaz031)
Do it in pencil first.
...but remember to ink in later. I e-mailed Edexcel about working on the pre-printed grids in pencil only on D1, since an error in ink could screw it all up, and they said you must not answer in pencil. Only graphs are allowed in pencil.
Aitch
14. you must not answer in pencil. Only graphs are allowed in pencil.
Aitch
But we're talking about graphs?
Thanks for mentioning that though, it is important to make sure the final version is in pen.
15. (Original post by Gaz031)
But we're talking about graphs?
Thanks for mentioning that though, it is important to make sure the final version is in pen.
I think the message is all text in ink!

Aitch
16. i always do it is pencil and never ink. Mistakes are permanent as u cannot use tipex but worse, when u accidentally highlighted the wrong saturated arc.
17. Two more problems...

What is the whole backflow thing in flow networks, im not sure i understand. But i have got a question wrong because of not knowing so i think it'll come up in the exam.

With bipartite graphs, is it necessary to start the maximum matching algorithm in column X?? can you keep going with alternating path, to complete the matchingm, even though u have found a breakthrough? or do you have to stop and start again?

Btw, the simplex method has come up in every edexcel paper so far.
18. r u sure???? it ddnt come up in jan 2005, or jan 2004 or jan2003, but has com up in nearly all june papers
19. Ok well maybe they dont all have the simplex method in them, sorry.

However i know nearly all the solomon papers have simplex in, and they normally prove pretty acurate for the types of questions you'll face.

There is a link somewhere in this forum for D1 notes... that summarises the simplex method quite well if you feel like learning it before the exam.
20. (Original post by samthemoron)
Ok well maybe they dont all have the simplex method in them, sorry.

However i know nearly all the solomon papers have simplex in, and they normally prove pretty acurate for the types of questions you'll face.

There is a link somewhere in this forum for D1 notes... that summarises the simplex method quite well if you feel like learning it before the exam.

This is useful:

http://www.colchsfc.ac.uk/maths/simp...dexcelsimp.htm

Aitch

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