For which values of n is the Fibonacci number fn even? Which Fibonnaci numbers are divisible by 3?
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?”
What would you get if you continued this sequence of fraction sums? 1/2 + 2/1 = 2/3 + 3/2 = 3/4 + 4/3 =
Choose four consecutive whole numbers. Multiply the first and last numbers together. Multiply the middle pair together. What do you notice?
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 ...?
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?
Many numbers can be expressed as the difference of two perfect squares. What do you notice about the numbers you CANNOT make?
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?
What is the value of the integers a and b where sqrt(8-4sqrt3) = sqrt a - sqrt b?
Given a set of points (x,y) with distinct x values, find a polynomial that goes through all of them, then prove some results about the existence and uniqueness of these polynomials.
Some students have been working out the number of strands needed for different sizes of cable. Can you make sense of their solutions?
What is the total number of squares that can be made on a 5 by 5 geoboard?
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. . . .
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.
Pick a square within a multiplication square and add the numbers on each diagonal. What do you notice?
Sets of integers like 3, 4, 5 are called Pythagorean Triples, because they could be the lengths of the sides of a right-angled triangle. Can you find any more?
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. . . .
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.
Can you find the values at the vertices when you know the values on the edges?
Can you find the area of a parallelogram defined by two vectors?
Caroline and James pick sets of five numbers. Charlie chooses three of them that add together to make a multiple of three. Can they stop him?
Charlie likes tablecloths that use as many colours as possible, but insists that his tablecloths have some symmetry. Can you work out how many colours he needs for different tablecloth designs?
Can you use the diagram to prove the AM-GM inequality?
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.
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?
Generalise the sum of a GP by using derivatives to make the coefficients into powers of the natural numbers.
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.
If for any triangle ABC tan(A - B) + tan(B - C) + tan(C - A) = 0 what can you say about the triangle?
To avoid losing think of another very well known game where the patterns of play are similar.
An article which gives an account of some properties of magic squares.
Generalise this inequality involving integrals.
Can you explain the surprising results Jo found when she calculated the difference between square numbers?
Can you find the values at the vertices when you know the values on the edges of these multiplication arithmagons?
Jo made a cube from some smaller cubes, painted some of the faces of the large cube, and then took it apart again. 45 small cubes had no paint on them at all. How many small cubes did Jo use?
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.
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?
When if ever do you get the right answer if you add two fractions by adding the numerators and adding the denominators?
The opposite vertices of a square have coordinates (a,b) and (c,d). What are the coordinates of the other vertices?
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.
Equal touching circles have centres on a line. From a point of this line on a circle, a tangent is drawn to the farthest circle. Find the lengths of chords where the line cuts the other circles.
A game for 2 players
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.
Can you see how to build a harmonic triangle? Can you work out the next two rows?
Use the animation to help you work out how many lines are needed to draw mystic roses of different sizes.
Can you work out the irrational numbers that belong in the circles to make the multiplication arithmagon correct?
Charlie has moved between countries and the average income of both has increased. How can this be so?
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 collection of games on the NIM theme
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.