
Choose any two numbers. Call them a and b. Work out the arithmetic mean and the geometric mean. Which is bigger? Repeat for other pairs of numbers. What do you notice?
An article which gives an account of some properties of magic squares.

Janine noticed, while studying some cube numbers, that if you take three consecutive whole numbers and multiply them together and then add the middle number of the three, you get the middle number. . . .

Given that a, b and c are natural numbers show that if sqrt a+sqrt b is rational then it is a natural number. Extend this to 3 variables.
Fractional calculus is a generalisation of ordinary calculus where you can differentiate n times when n is not a whole number.

When if ever do you get the right answer if you add two fractions by adding the numerators and adding the denominators?

If for any triangle ABC tan(A - B) + tan(B - C) + tan(C - A) = 0 what can you say about the triangle?

Generalise the sum of a GP by using derivatives to make the coefficients into powers of the natural numbers.

Colour the squares of the square tablecloth so that each square is the same colour as all the symmetrically placed squares and a different colour from the rest of the squares.
An account of some magic squares and their properties and and how to construct them for yourself.

Jo has three numbers which she adds together in pairs. When she does this she has three different totals: 11, 17 and 22 What are the three numbers Jo had to start with?”

The diagram illustrates the formula: 1 + 3 + 5 + ... + (2n - 1) = n² Use the diagram to show that any odd number is the difference of two squares.

First of all, pick the number of times a week that you would like to eat chocolate. Multiply this number by 2...

Problem solving is at the heart of the NRICH site. All the problems give learners opportunities to learn, develop or use mathematical concepts and skills. Read here for more information.

Can you show that you can share a square pizza equally between two people by cutting it four times using vertical, horizontal and diagonal cuts through any point inside the square?

Start with any number of counters in any number of piles. 2 players take it in turns to remove any number of counters from a single pile. The winner is the player to take the last counter.

An article for teachers and pupils that encourages you to look at the mathematical properties of similar games.

Many numbers can be expressed as the difference of two perfect squares. What do you notice about the numbers you CANNOT make?

Build gnomons that are related to the Fibonacci sequence and try to explain why this is possible.

A game for 2 players. Set out 16 counters in rows of 1,3,5 and 7. Players take turns to remove any number of counters from a row. The player left with the last counter looses.

The triangle OMN has vertices on the axes with whole number co-ordinates. How many points with whole number coordinates are there on the hypotenuse MN?

This is a Dutch game for two players. It will test your powers of shape and space visualisation

A game for 2 players with similaritlies to NIM. Place one counter on each spot on the games board. Players take it is turns to remove 1 or 2 adjacent counters. The winner picks up the last counter.

For which values of n is the Fibonacci number fn even? Which Fibonnaci numbers are divisible by 3?

If a is the radius of the axle, b the radius of each ball-bearing, and c the radius of the hub, why does the number of ball bearings n determine the ratio c/a? Find a formula for c/a in terms of n.

Your data is a set of positive numbers. What is the maximum value that the standard deviation can take?

What is the value of the integers a and b where sqrt(8-4sqrt3) = sqrt a - sqrt b?

The aim of the game is to slide the green square from the top right hand corner to the bottom left hand corner in the least number of moves.

The incircles of 3, 4, 5 and of 5, 12, 13 right angled triangles have radii 1 and 2 units respectively. What about triangles with an inradius of 3, 4 or 5 or ...?

The opposite vertices of a square have coordinates (a,b) and (c,d). What are the coordinates of the other vertices?

Investigate the relationship between the coordinates of the endpoint of a line through the origin and the number of grid squares it crosses.

Can you explain the surprising results Jo found when she calculated the difference between square numbers?

Triangle ABC is an equilateral triangle with three parallel lines going through the vertices. Calculate the length of the sides of the triangle if the perpendicular distances between the parallel. . . .
Here explore some ideas of how the definitions and methods of calculus change if you integrate or differentiate n times when n is not a whole number.

If you continue the pattern, can you predict what each of the following areas will be? Try to explain your prediction.

Start with any number of counters in any number of piles. 2 players take it in turns to remove any number of counters from a single pile. The loser is the player who takes the last counter.

Make and prove a conjecture about the cyclic quadrilateral inscribed in a circle of radius r that has the maximum perimeter and the maximum area.
This article for teachers describes several games, found on the site, all of which have a related structure that can be used to develop the skills of strategic planning.
You can differentiate and integrate n times but what if n is not a whole number? This generalisation of calculus was introduced and discussed on askNRICH by some school students.

What is the ratio of the area of a square inscribed in a semicircle to the area of the square inscribed in the entire circle?

A counter is placed in the bottom right hand corner of a grid. You toss a coin and move the star according to the following rules: ... What is the probability that you end up in the top left-hand. . . .

Show that for any triangle it is always possible to construct 3 touching circles with centres at the vertices. Is it possible to construct touching circles centred at the vertices of any polygon?

What is the volume of the solid formed by rotating this right angled triangle about the hypotenuse?

ABC and DEF are equilateral triangles of side 3 and 4 respectively. Construct an equilateral triangle whose area is the sum of the area of ABC and DEF.
This article by Alex Goodwin, age 18 of Madras College, St Andrews describes how to find the sum of 1 + 22 + 333 + 4444 + ... to n terms.

These gnomons appear to have more than a passing connection with the Fibonacci sequence. This problem ask you to investigate some of these connections.

Can you find a general rule for finding the areas of equilateral triangles drawn on an isometric grid?