Start with a triangle. Can you cut it up to make a rectangle?

Determine the total shaded area of the 'kissing triangles'.

Liethagoras, Pythagoras' cousin (!), was jealous of Pythagoras and came up with his own theorem. Read this article to find out why other mathematicians laughed at him.

A description of some experiments in which you can make discoveries about triangles.

Prove that a triangle with sides of length 5, 5 and 6 has the same area as a triangle with sides of length 5, 5 and 8. Find other pairs of non-congruent isosceles triangles which have equal areas.

Imagine an infinitely large sheet of square dotty paper on which you can draw triangles of any size you wish (providing each vertex is on a dot). What areas is it/is it not possible to draw?

ABCDEFGH is a 3 by 3 by 3 cube. Point P is 1/3 along AB (that is AP : PB = 1 : 2), point Q is 1/3 along GH and point R is 1/3 along ED. What is the area of the triangle PQR?

Draw all the possible distinct triangles on a 4 x 4 dotty grid. Convince me that you have all possible triangles.

If you know the sizes of the angles marked with coloured dots in this diagram which angles can you find by calculation?

An irregular tetrahedron is composed of four different triangles. Can such a tetrahedron be constructed where the side lengths are 4, 5, 6, 7, 8 and 9 units of length?

Draw three straight lines to separate these shapes into four groups - each group must contain one of each shape.

A floor is covered by a tessellation of equilateral triangles, each having three equal arcs inside it. What proportion of the area of the tessellation is shaded?

Draw some isosceles triangles with an area of $9$cm$^2$ and a vertex at (20,20). If all the vertices must have whole number coordinates, how many is it possible to draw?

Can you dissect an equilateral triangle into 6 smaller ones? What number of smaller equilateral triangles is it NOT possible to dissect a larger equilateral triangle into?

Triangles are formed by joining the vertices of a skeletal cube. How many different types of triangle are there? How many triangles altogether?

You have pitched your tent (the red triangle) on an island. Can you move it to the position shown by the purple triangle making sure you obey the rules?

Make an equilateral triangle by folding paper and use it to make patterns of your own.

ABC is an equilateral triangle and P is a point in the interior of the triangle. We know that AP = 3cm and BP = 4cm. Prove that CP must be less than 10 cm.

Can you help the children find the two triangles which have the lengths of two sides numerically equal to their areas?

Have you noticed that triangles are used in manmade structures? Perhaps there is a good reason for this? 'Test a Triangle' and see how rigid triangles are.

The graph below is an oblique coordinate system based on 60 degree angles. It was drawn on isometric paper. What kinds of triangles do these points form?

Use the interactivity to investigate what kinds of triangles can be drawn on peg boards with different numbers of pegs.

Make a flower design using the same shape made out of different sizes of paper.

Can you sketch triangles that fit in the cells in this grid? Which ones are impossible? How do you know?

Generate three random numbers to determine the side lengths of a triangle. What triangles can you draw?

What is the total area of the four outside triangles which are outlined in red in this arrangement of squares inside each other?

The large rectangle is divided into a series of smaller quadrilaterals and triangles. Can you untangle what fractional part is represented by each of the shapes?

Can you work out what shape is made by folding in this way? Why not create some patterns using this shape but in different sizes?

Cut a square of paper into three pieces as shown. Now,can you use the 3 pieces to make a large triangle, a parallelogram and the square again?

This problem challenges you to work out what fraction of the whole area of these pictures is taken up by various shapes.

The triangles in these sets are similar - can you work out the lengths of the sides which have question marks?

How would you move the bands on the pegboard to alter these shapes?

Using different numbers of sticks, how many different triangles are you able to make? Can you make any rules about the numbers of sticks that make the most triangles?

The ancient Egyptians were said to make right-angled triangles using a rope with twelve equal sections divided by knots. What other triangles could you make if you had a rope like this?

This interactivity allows you to sort logic blocks by dragging their images.

This article for teachers suggests activities based on pegboards, from pattern generation to finding all possible triangles, for example.

Can you make the most extraordinary, the most amazing, the most unusual patterns/designs from these triangles which are made in a special way?

Can you each work out what shape you have part of on your card? What will the rest of it look like?

I cut this square into two different shapes. What can you say about the relationship between them?

This practical problem challenges you to create shapes and patterns with two different types of triangle. You could even try overlapping them.

In how many ways can you fit two of these yellow triangles together? Can you predict the number of ways two blue triangles can be fitted together?

A game in which players take it in turns to turn up two cards. If they can draw a triangle which satisfies both properties they win the pair of cards. And a few challenging questions to follow...

A game in which players take it in turns to try to draw quadrilaterals (or triangles) with particular properties. Is it possible to fill the game grid?

Use the interactivity to find all the different right-angled triangles you can make by just moving one corner of the starting triangle.

What do these two triangles have in common? How are they related?

Can you find all the different triangles on these peg boards, and find their angles?

How many different triangles can you make on a circular pegboard that has nine pegs?