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OCR (not MEI) D1 24th May 2013

Hi guys.
Who is taking this exam in about 2 weeks? :biggrin:
How is your revision going? How do you feel about the exam?

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Reply 1
Original post by purplemind
Hi guys.
Who is taking this exam in about 2 weeks? :biggrin:
How is your revision going? How do you feel about the exam?


I am, such an interesting module despite its negative reputation.

I have attached the Jan 13 paper and mark scheme.
(edited 10 years ago)
Reply 2
Original post by metaltron
I am, such an interesting module despite its negative reputation.

I have attached the Jan 13 D1 paper and MS.


Well, it isn't so bad. :smile:
I just don't like is when they give you a question when you are supposed to explain for example why it isn't possible to have a graph with orders 1, 2, 3, 3, 4, etc.
What makes me sick is decimals in Simplex questions. When I get 'weird' decimals, ie. different than 0.1-0.9 I think I have done something wrong maybe. :tongue:
Reply 3
I have this and am doing it in place of an FP2 retake. Need to do well in this to meet my offer! I find that some of the questions are a bit reminiscent of GCSE science Qs, as in the markscheme doesn't allow answers which are correct! Other than that it is interesting in the main part but the algorithms part can only be described as "crushingly boring"!
Reply 4
Original post by purplemind
Well, it isn't so bad. :smile:
I just don't like is when they give you a question when you are supposed to explain for example why it isn't possible to have a graph with orders 1, 2, 3, 3, 4, etc.
What makes me sick is decimals in Simplex questions. When I get 'weird' decimals, ie. different than 0.1-0.9 I think I have done something wrong maybe. :tongue:


Hey, Yeah revision is going alright i suppose. Do you want to do past paper questions together? Im kind of panicking about the exam so any support would be much appreciated!

thanks :smile:
PM me

Oh and does anyone have the jan 2013 paper?
Reply 5
Original post by kabutsu12
I have this and am doing it in place of an FP2 retake. Need to do well in this to meet my offer! I find that some of the questions are a bit reminiscent of GCSE science Qs, as in the markscheme doesn't allow answers which are correct! Other than that it is interesting in the main part but the algorithms part can only be described as "crushingly boring"!


The algorithms are the best part. If you looks deeper into them, you'll find the maths eventually (apart from nearest neighbour, but hey that's easy :tongue: )
I'm doing this exam! Unfortunately I've got S1 on the same day as well!
Reply 7
Original post by JASApplications
I'm doing this exam! Unfortunately I've got S1 on the same day as well!

So do I so I know how it feels. :smile:
Reply 8
So question 4 v on this paper http://www.ocr.org.uk/Images/61395-question-paper-unit-4736-01-decision-mathematics-1.pdf asks you to find the lower bound for the travelling salesperson problem. The algorithm as written in my CGP text book says "find the lowest weight edges joined to the vetex you delete" That is what I did in the question using the graph not the matrix and I got it wrong. I still think I'm right. Pretty confused about this, help appreciated :smile:
Reply 9
Original post by kabutsu12
So question 4 v on this paper http://www.ocr.org.uk/Images/61395-question-paper-unit-4736-01-decision-mathematics-1.pdf asks you to find the lower bound for the travelling salesperson problem. The algorithm as written in my CGP text book says "find the lowest weight edges joined to the vetex you delete" That is what I did in the question using the graph not the matrix and I got it wrong. I still think I'm right. Pretty confused about this, help appreciated :smile:

First, you delete row and column G, so you are left with A, B, C, D, E, F. You apply Prim's to the matrix you are left with ie. after deleting G, and start at B. You get the weight of minimum spanning tree. Then, you add weight of 2 shortest edges from G to the weight of minimum spanning tree to get the lower bound.
Reply 10
Yes, but the arc D-G doesn't exist on the graph! only as a shortest route (so not strictly an arc) in a matrix so why not the 0.6 and 0.45 that are ​connected as arcs?
Reply 11
Original post by kabutsu12
Yes, but the arc D-G doesn't exist on the graph! only as a shortest route (so not strictly an arc) in a matrix so why not the 0.6 and 0.45 that are ​connected as arcs?

You use matrix to build a spanning tree. You don't include G in a spanning tree because it is a special node, and I don't see why you should be overly worried. :smile:
DG doesn't exist but D can be connected to G via F which gives the shortest route from D to G which is 0.45+0.2=0.65 that is why they put it in a matrix.
And, they asked you to use the martix, not the graph, so you just use it.
Anyway, 2 shortest arcs from G are 0.45 and 0.6 which do exist on a graph and you use them for your lower bound (and the spanning tree you have found from a matrix after deleting G).
(edited 10 years ago)
Reply 12
Ahh, the thing I'm confused with is that I did you use 0.45 and 0.65 for for my lower bound and got it wrong! The mark scheme says it's 0.45 +0.65 + the weight of the MST for the lower bound.
Reply 13
Original post by kabutsu12
Ahh, the thing I'm confused with is that I did you use 0.45 and 0.65 for for my lower bound and got it wrong! The mark scheme says it's 0.45 +0.65 + the weight of the MST for the lower bound.

Oh, I know what you mean now (first I only looked at weights, not what is connected to what. :tongue:)
It's because 0.6 and 0.45 both join F to G. And 0.45 is the shorter one of the 2.
So 0.65 (which is the next shortest) would be given by D G via F.
(edited 10 years ago)
Reply 14
That bit doesn't make sense as it's not in keeping with the algorithm in all textbooks!
Reply 15
It doesn't if you look at the previous graph.
But imagine you have no graph and it is a matrix representing another graph which contains all the arcs instead of 'combined arcs'. :smile:
So just forget the graph, use the matrix if they ask to do so.
Reply 16
Sure thing :smile: The way the question was worded really didn't lead onto that!
Reply 17
Original post by bellaanne
Hey, Yeah revision is going alright i suppose. Do you want to do past paper questions together? Im kind of panicking about the exam so any support would be much appreciated!

thanks :smile:
PM me

Oh and does anyone have the jan 2013 paper?

I think you sent me pm about Q5 (vi) before. I gave it a go. :smile: First I had to do previous parts, so there is my full solution.

Spoiler

Reply 18
Jan 13 paper would be useful!
Reply 19
Original post by kabutsu12
Jan 13 paper would be useful!

Look at the second post on the 1st page. :smile:

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