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tic tac toe 2

Game Theory and Tic Tac Toe

The game of Noughts and Crosses or Tic Tac Toe is well known throughout the world and variants are thought to have been played over 2000 years ago in Rome. It’s a very simple game – the first person to get 3 in a row wins. In fact it’s so simple that it has been “solved” – before any move has been played we already know it should result in a draw (as long as the participants play optimal moves).

The way to solve Noughts and Crosses is to use combinatorial Game Theory – which is a branch of mathematics that allows us to analyses all different outcomes of an event.

tic tac toe

This is the start of the game tree for Noughts and Crosses. We can expand this game tree to cover every possible outcome for the game. Once this complete tree is drawn, any participant can work through this tree to see what is their optimal move at any one time from any position.

An upper bound for the number of positions and number of different games is given by:

3 9= 19,683.  This is the total number of possible game positions in a 3×3 grid – as every square will either be a O, X or blank.

9! = 362,880. This is the total number of ways that positions can be filled on the grid. (First you have 9 choices of squares, then there are 8 choices of squares etc).  This counts each X and O as distinct from other X and Os.

9 choose 5 = 126.  This is the number of different combinations of filling the grid with 5 Xs and 4 Os.

However the analysis of this game tree can be significantly simplified by realising that many different positions are simply reflections or rotations of each other. By looking only for distinct positions (positions that are isometric under refection and rotation) we can, for example, see that there are actually only three distinct starting moves – as shown in the diagram above.

James Grime from Numberphile takes us through how to answer a related question, “How many different ways are there to completely fill a Noughts and Crosses board with 5 Xs and 4 Os – not including rotations and reflections?”  The solution above is a little complicated (it makes uses of Group Theory) but it is an excellent introduction to some uses of higher level mathematics.

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This somewhat horrendous looking graphic actually contains the solution to playing Noughts and Crosses. You can use it to always achieve the optimal outcome for X. It works as follows:
1) The big red X in the top left hand corner represents your best first move. So you make this move first.
2) Next, you see what your opponent does and choose the grid with the big black O in the position they have chosen.
3) This new grid will have a big red X – this is your next optimal move.
4) You then remain in your subsection of the larger grid – and repeat the process.

If you liked this you might also like:

Game Theory and Evolution – do nice guys always finish last?

Knight’s Tour – an exploration of this 1000 year old mathematical puzzle.

IB Revision

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If you’re already thinking about your coursework then it’s probably also time to start planning some revision, either for the end of Year 12 school exams or Year 13 final exams. There’s a really great website that I would strongly recommend students use – you choose your subject (HL/SL/Studies if your exam is in 2020 or Applications/Analysis if your exam is in 2021), and then have the following resources:

Screen Shot 2018-03-19 at 4.42.05 PM.pngThe Questionbank takes you to a breakdown of each main subject area (e.g. Algebra, Calculus etc) and each area then has a number of graded questions. What I like about this is that you are given a difficulty rating, as well as a mark scheme and also a worked video tutorial.  Really useful!

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The Practice Exams section takes you to ready made exams on each topic – again with worked solutions.  This also has some harder exams for those students aiming for 6s and 7s and the Past IB Exams section takes you to full video worked solutions to every question on every past paper – and you can also get a prediction exam for the upcoming year.

I would really recommend everyone making use of this – there is a mixture of a lot of free content as well as premium content so have a look and see what you think.

Website Stats


IB HL Paper 3 Practice Questions (120 page pdf)

IB HL Paper 3 Practice Questions 

Seventeen full investigation questions – each one designed to last around 1 hour, and totaling around 40 pages and 600 marks worth of content.  There is also a fully typed up mark scheme.  Together this is around 120 pages of content.

Available to download here.

IB Maths Exploration Guide

IB Maths Exploration Guide

A comprehensive 63 page pdf guide to help you get excellent marks on your maths investigation. Includes:

  1. Investigation essentials,
  2. Marking criteria guidance,
  3. 70 hand picked interesting topics
  4. Useful websites for use in the exploration,
  5. A student checklist for top marks
  6. Avoiding common student mistakes
  7. A selection of detailed exploration ideas
  8. Advice on using Geogebra, Desmos and Tracker.

Available to download here.

Modelling Guide

IB Exploration Modelling Guide 

A 50 page pdf guide full of advice to help with modelling explorations – focusing in on non-calculator methods in order to show good understanding.

Modelling Guide includes:

Linear regression and log linearization, quadratic regression and cubic regression, exponential and trigonometric regression, comprehensive technology guide for using Desmos and Tracker.

Available to download here.

Statistics Guide

IB Exploration Statistics Guide

A 55 page pdf guide full of advice to help with modelling explorations – focusing in on non-calculator methods in order to show good understanding.

Statistics Guide includes: Pearson’s Product investigation, Chi Squared investigation, Binomial distribution investigation, t-test investigation, sampling techniques, normal distribution investigation and how to effectively use Desmos to represent data.

Available to download here.

IB Revision Notes

IB Revision Notes

Full revision notes for SL Analysis (60 pages), HL Analysis (112 pages) and SL Applications (53 pages).  Beautifully written by an experienced IB Mathematics teacher, and of an exceptionally high quality.  Fully updated for the new syllabus.  A must for all Analysis and Applications students!

Available to download here.

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