Yatir from Israel describes his method for summing a series of triangle numbers.

Make a conjecture about the curved track taken by the complex roots of a quadratic equation and use complex conjugates to prove your conjecture.

Yatir from Israel wrote this article on numbers that can be written as $ 2^n-n $ where n is a positive integer.

If a number N is expressed in binary by using only 'ones,' what can you say about its square (in binary)?

Make a conjecture about the sum of the squares of the odd positive integers. Can you prove it?

Find the maximum value of n to the power 1/n and prove that it is a maximum.

Find the sum, f(n), of the first n terms of the sequence: 0, 1, 1, 2, 2, 3, 3........p, p, p +1, p + 1,..... Prove that f(a + b) - f(a - b) = ab.

Can you find the values at the vertices when you know the values on the edges of these multiplication arithmagons?

Investigate sequences given by $a_n = \frac{1+a_{n-1}}{a_{n-2}}$ for different choices of the first two terms. Make a conjecture about the behaviour of these sequences. Can you prove your conjecture?

Drawing a triangle is not always as easy as you might think!

What have Fibonacci numbers got to do with Pythagorean triples?

This group tasks allows you to search for arithmetic progressions in the prime numbers. How many of the challenges will you discover for yourself?

Join the midpoints of a quadrilateral to get a new quadrilateral. What is special about it?

The points P, Q, R and S are the midpoints of the edges of a non-convex quadrilateral.What do you notice about the quadrilateral PQRS and its area?

Change the squares in this diagram and spot the property that stays the same for the triangles. Explain...

A point moves on a line segment. A function depends on the position of the point. Where do you expect the point to be for a minimum of this function to occur.

How many different colours of paint would be needed to paint these pictures by numbers?

What have Fibonacci numbers to do with solutions of the quadratic equation x^2 - x - 1 = 0 ?

A and B are two fixed points on a circle and RS is a variable diamater. What is the locus of the intersection P of AR and BS?

Find out about Magic Squares in this article written for students. Why are they magic?!

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

Evaluate these powers of 67. What do you notice? Can you convince someone what the answer would be to (a million sixes followed by a 7) squared?

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

A sequence of polynomials starts 0, 1 and each poly is given by combining the two polys in the sequence just before it. Investigate and prove results about the roots of the polys.

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. . . .

Steve has created two mappings. Can you figure out what they do? What questions do they prompt you to ask?

Here is a machine with four coloured lights. Can you make two lights switch on at once? Three lights? All four lights?

Make and prove a conjecture about the value of the product of the Fibonacci numbers $F_{n+1}F_{n-1}$.

Beautiful mathematics. Two 18 year old students gave eight different proofs of one result then generalised it from the 3 by 1 case to the n by 1 case and proved the general result.

A polite number can be written as the sum of two or more consecutive positive integers. Find the consecutive sums giving the polite numbers 544 and 424. What characterizes impolite numbers?

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.

A serious but easily readable discussion of proof in mathematics with some amusing stories and some interesting examples.

In 15 years' time my age will be the square of my age 15 years ago. Can you work out my age, and when I had other special birthdays?

Have a go at creating these images based on circles. What do you notice about the areas of the different sections?

Can you find some Pythagorean Triples where the two smaller numbers differ by 1?

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 find a rule which connects consecutive triangular numbers?

Can you find a rule which relates triangular numbers to square numbers?

Pick a square within a multiplication square and add the numbers on each diagonal. What do you notice?

Take any pair of two digit numbers x=ab and y=cd where, without loss of generality, ab > cd . Form two 4 digit numbers r=abcd and s=cdab and calculate: {r^2 - s^2} /{x^2 - y^2}.

A introduction to how patterns can be deceiving, and what is and is not a proof.

Show that all pentagonal numbers are one third of a triangular number.

Starting with one of the mini-challenges, how many of the other mini-challenges will you invent for yourself?

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?

Use the computer to model an epidemic. Try out public health policies to control the spread of the epidemic, to minimise the number of sick days and deaths.