D1 (Decision 1) 17 May 2013 Official Thread

OP

Original post by zmai

Haha no I am being serious! I might be on a different exam board to you! But I learnt that particularly for D1 it's quality not quantity...if you don't finish at least try and get all the marks for the ones you can do/have time I do! And the year before me they made one of those impossible questions so really don't get too hung up about it! (:

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Ah kk thanks alot, yeah that's what I am learning slowly as I do more Practice Questions (for all my subjects) - It's better to get all the marks for the question your doing rather than rushing.

I'll remember to keep calm and carry on. Cheers buddy

Reply 82

coolheadjody

5

Thanks maths247, you've probably helped me more than my teachers. Don't know what I would do without your channel. However I watched your linear programming video; the 40 min one, and you don't really address this problem, you just state it and that's why I got confused. However keep up the channel, it's a godsend. The best maths channel on YouTube.

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Reply 83

OP

Original post by This Excellency

Seems like it. Look at the answer for Exercise 5B Q5. It's similar to the one you posted.

Told ya you could do it :wink:

Thanks alot mate.

If anyone wants to join me I've done Practice Papers A and B so far (B, and D respectively) and I'm doing the Mock Paper and Specimen Paper this weekend?

How far is everyone so far?

Reply 84

m4ths/maths247

7

Original post by coolheadjody

Thanks maths247, you've probably helped me more than my teachers. Don't know what I would do without your channel. However I watched your linear programming video; the 40 min one, and you don't really address this problem, you just state it and that's why I got confused. However keep up the channel, it's a godsend. The best maths channel on YouTube.

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Thanks for the positive words.
I have chucked the January 2012 paper out too now to try and help.
Good luck to all those sitting the exam.

Reply 85

OP

Found a link:

http://clc2.uniservity.com/GroupWorkspaces.asp?GroupID=320323&WorkspaceID=397371&Javascript=False

Reply 86

OP

Definitions for the D1 Exam (and what they really mean!)

Graphs
A graph G consists of points (vertices or nodes) which are connected by lines (edges or arcs). A graph is just some points joined together with lines.

A subgraph of G is a graph, each of whose vertices belongs to G and each of whose edges belongs to G. A subgraph is just part of a bigger graph.

If a graph has a number associated with each edge (usually called its weight) then the graph is called a weighted graph or network. A weighted graph is one with values, eg distance, assigned to the edges.

The degree or valency of a vertex is the number of edges incident to it. A vertex is odd (even) if it has odd (even) degree. The degree of a vertex is how many edges go into it.

A path is a finite sequence of edges, such that the end vertex of one edge in the sequence is the start vertex of the next, and in which no vertex appears more than once. A path describes a ‘journey’ around a graph, where no vertex is visited more than once.

A walk is a path where you may visit vertices more than once.

A cycle (circuit) is a closed path, i.e. the end vertex of the last edge is the start vertex of the first edge.

Two vertices are connected if there is a path between them. A graph is connected if all its vertices are connected. Two vertices are connected if you can walk along edges from one to the othe, in a connected graph all vertices are connected.

If the edges of a graph have a direction associated with them they are known as directed edges and the graph is known as a digraph. A digraph will have arrows on the edges.

A tree is a connected graph with no cycles.

A spanning tree of a graph G is a subgraph which includes all the vertices of G and is also a tree.

A minimum spanning tree (MST) is a spanning tree such that the total length of its arcs is as small as possible. (MST is sometimes called a minimum connector.)

A graph in which each of the n vertices is connected to every other vertex is called a complete graph. In a complete graph every vertex is directly connected to every other edge.

Matchings

A bipartite graph consists of two sets of vertices X and Y. The edges only join vertices in X to vertices in Y, not vertices within a set. (If there are r vertices in X and s vertices in Y then this graph is Kr,s.) A bipartite graph has two sets of nodes. Nodes of one set can only be matched to nodes of the other set.

A matching is the pairing of some or all of the elements of one set, X, with elements of a second set, Y. If every member of X is paired with a member of Y the matching is said to be a complete matching. In a matching some of one set are joined to the other. In a complete matching they’re all joined.

An Alternating Path starts at an unconnected vertex in one set and ends at an unconnected vertex in the other set. Edges alternate between those ‘not in’ and ‘in’ the initial matching.

Reply 87

OP

ANY HELP???

I keep getting Kruskals questions wrong. I don't know whether to Reject if a Node is already in the Minimum Spanning Tree you are building???

why Kruskals.png

Reply 88

posthumus

20

Original post by Better

ANY HELP???

I keep getting Kruskals questions wrong. I don't know whether to Reject if a Node is already in the Minimum Spanning Tree you are building???

why Kruskals.png

You reject FH because it creates a loop (please correct my use of the term 'loop').

I think you just forgot to highlight EF (28<30), I'm not sure ? :confused:

Reply 89

OP

Original post by posthumus

You reject FH because it creates a loop (please correct my use of the term 'loop').

I think you just forgot to highlight EF (28<30), I'm not sure ? :confused:

Ah perfect yeah thank you, I watched a Video too on Maths24/7, I was doing Kruskals wrong. Next time I will just order ALL of the arcs, from lowest to highest and cross or tick them all.

I don't really work it out too clearly.

Reply 90

Westeros

12

Hey, can someone help me with a Linear Programming question from the Edexcel D1 January 2008 paper, here's the link if you don't have the paper
http://www.edexcel.com/migrationdocuments/QP%20GCE%20Curriculum%202000/January%202008/6689_01_que_20080111.pdf

It's question 7, I've done part a, b and c - but I'm stuck on d. How do I know what the profit line is? Or is it easier to test points. I've looked at the mark scheme and it says the answer is 80. I just don't understand! Can someone help me please :confused:

Reply 91

ScarlettFierce

2

I HATE D1! Good luck guys I feel your pain!

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Reply 92

can someone please help me on question 3 on page 145 mixed exercise 6E

many thanks

Reply 93

coolheadjody

5

Yh i don't get it myself

Reply 94

Original post by coolheadjody

Yh i don't get it myself

do you get question2)b) on page 77, I have no idea why they finished at A when in question stated finishing at C

Reply 95

coolheadjody

5

I get the first part why it's 9x+4y is less than or equal to 72, but the others I don't.

Reply 96

Westeros

12

Original post by otrivine

can someone please help me on question 3 on page 145 mixed exercise 6E

many thanks

What part don't you understand? For the first part about the inequalities proof:

i) 54x + 24y ≥ 432
Then divide through by 6 to give you:
9x + 4y ≥ 72 (This shows the total amount of people, 408 pupils+28adults)

ii) x + y ≤ 12
This shows the total number of adults - 2 adults per coach so half the 24 to give you 12

iii) x ≤ 7
This shows the number of large coaches, the company only has 7 large coaches

For part c) Minimise C = 336x + 252y
Here you are minimising the cost, and you can factories it to 84(4x+3y)

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Reply 97

Original post by Westeros

What part don't you understand? For the first part about the inequalities proof:

i) 54x + 24y ≥ 432
Then divide through by 6 to give you:
9x + 4y ≥ 72 (This shows the total amount of people, 408 pupils+28adults)

ii) x + y ≤ 12
This shows the total number of adults - 2 adults per coach so half the 24 to give you 12

iii) x ≤ 7
This shows the number of large coaches, the company only has 7 large coaches

For part c) Minimise C = 336x + 252y
Here you are minimising the cost, and you can factories it to 84(4x+3y)

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thank you and what about question 2)b) page 77 , why did they finish at point A?

Reply 98

Westeros

12

Original post by otrivine

do you get question2)b) on page 77, I have no idea why they finished at A when in question stated finishing at C

The Edexcel Textbook has loads of stupid mistakes in it. There are loads of different paths you can take, you don't need to have the exact same one as the answers in the back of the book. As long as your route has 23 letters in, each road/arc is traversed twice and length is 12km then it's more than likely correct! I wouldn't worry too much about it :-)

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Reply 99