Mathematics
Extreme Principle

Extreme Principle

Theory

The Extreme Principle is a problem-solving method where you focus on the largest, smallest, or otherwise most “extreme” element in a set to gain insight into the problem. By examining such an element, you can often find a contradiction, reduce the complexity, or uncover a key property that must hold.

For example, if you are trying to prove that a certain configuration is impossible, you might start by assuming it exists and then pick the “extreme” case — the point farthest to the left, the smallest number, the shortest segment — and show that this choice leads to a contradiction. In geometry and combinatorics, the Extreme Principle often helps identify hidden constraints that are hard to see when looking at the problem as a whole.

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Problems

  1. Is it possible to mark 100 points on a plane so that each marked point is the midpoint of the segment connecting two other marked points? What about in space?
  2. The sum of positive numbers x₁, x₂, …, x₁₀₀ is equal to 1. Prove the inequality x₁x₂ + x₂x₃ + … + x₉₉x₁₀₀ ≤ 1/4.
  3. Prove that there is no convex polyhedron in which all faces have a different number of sides.
  4. On the plane, 100 straight lines are drawn. No two are parallel, and no three pass through the same point. Into how many regions do the lines divide the plane?
  5. a) Several identical coins are lying on a table without overlapping. Prove that there is a coin that touches no more than three others. b) There are 21 numbers, and the sum of any five of them is positive. Prove that the sum of all the numbers is positive.
  1. Six numbers are arranged around a circle, and each number is equal to the absolute value of the difference of the next two numbers in clockwise order. The sum of all the numbers is equal to 1. Find these numbers.
  2. Ali-Baba is trying to get into a cave. At the entrance stands a square table with a vessel in each corner. In each vessel there is a herring, which may be placed head-up or tail-up. From the outside, the positions of the herrings are not visible. Ali-Baba may put his hands into any two vessels, feel how the herrings are positioned, and set them however he likes (he may leave them as they were or flip one or both). This operation may be repeated several times. However, after each move the table is spun rapidly, so when it stops, it is impossible to tell which vessels were previously touched. The cave door opens if all herrings are in the same position. Help Ali-Baba find a strategy to enter the cave.
  3. Tower of Hanoi. The “Tower of Hanoi” puzzle consists of three pegs, with seven rings of decreasing size stacked on one of them. It is allowed to remove one ring at a time from any peg and place it on any other peg, but it is forbidden to place a larger ring on top of a smaller one. Is it possible, following these rules, to transfer all rings to another peg?
  4. On an infinite squared sheet of paper, 100 cells are coloured black and all others white. In one move, you may switch the colour of any four cells forming a 2×2 square. Prove that it is possible to make all cells white in several moves if and only if every row and every column contains an even number of black cells.
  5. In each cell of an infinite squared sheet of paper, a natural number is written. It turns out that each number is equal to the arithmetic mean of its four neighbouring numbers. Prove that all the numbers are equal to each other.
Ages
Parity
Worst Case Scenario
Racing
Percentages
Sums and averages
Equations
Pigeonhole principle
Productivity
Divisibility
Strategies in Games
River Crossing Riddles
Mathematics in Chess
Cryptarithms
Olympiad BIDMAS
Weighing Puzzles
Water Pouring Puzzles
Knights and Liars
Magic Square
Reductio ad impossibile or Proof by contradiction
Base Numbers Problems
Patterns in Numbers and Digits
Matchsticks
Invariant Principle
Method of Colouring
Induction Principle
Patterns
Symmetry
Heads and Feet
Ratio
Number Theory
Combinatorics (Primary)
Gauss's Method and the Sum of Consecutive Numbers
Paths
How many shapes?
Maximum and Minimum
Exponents (Last Digit)
Rationalily
Triangle Inequality
GCD and LCM
Extreme Principle
Remainders of Squares and Cubes

FAQ

Find quick answers to common questions about our lessons, pricing, scheduling, and how Exact Science can help your child excel.
Where do you hold your classes?
We hold our classes online or on-site on Saturdays at our branch in Pimlico Academy, London.
You can find our timetable here.
What do you need to start learning online?
For lessons you only need a computer or phone with a microphone, camera and Internet access. Wherever you are - in London, Nottingham, New York or Bali - online lessons will be at hand.
When can I take the trial lesson?
You can get acquainted with the school at any time convenient for you. To do this, just leave a request and sign up for a lesson.
What should I expect from the trial lesson?
The trial lesson is a 30-minute online session designed to get a sense of how your child approaches mathematical thinking and problem solving. (In practice, it often runs a bit longer if the student is engaged!)

We typically explore a range of fun and challenging problems drawn from competitions. We adapt the difficulty based on how the student responds, aiming to make it both accessible and stimulating.

After the session, we’ll have a quick conversation with the parent to share observations and suggest a personalised path forward.
I can't attend class, what should I do?
It is OK, it happens! Students have the opportunity to cancel a lesson up to 8 hours before the scheduled time without loss of payment. So you can reschedule it for a convenient time, and the teacher will have the opportunity to
I don't have much free time, will I have time to study?
Learning can take place at your own pace. We will select a convenient schedule and at any time we will help you change the schedule, take a break or adjust the program.
How long is one lesson?
All classes last 1 hour.

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Our teachers will tell you how to prepare for exams, help you cope with difficult tasks and win the Olympiad
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Founder &
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