Construct two equilateral triangles on a straight line. There are two lengths that look the same - can you prove it?
It is impossible to trisect an angle using only ruler and compasses but it can be done using a carpenter's square.
An equilateral triangle is constructed on BC. A line QD is drawn, where Q is the midpoint of AC. Prove that AB // QD.
Prove that the internal angle bisectors of a triangle will never be perpendicular to each other.
Draw some quadrilaterals on a 9-point circle and work out the angles. Is there a theorem?
This is the second article on right-angled triangles whose edge lengths are whole numbers.
Prove that, given any three parallel lines, an equilateral triangle always exists with one vertex on each of the three lines.
The first of two articles on Pythagorean Triples which asks how many right angled triangles can you find with the lengths of each side exactly a whole number measurement. Try it!
Semicircles are drawn on the sides of a rectangle. Prove that the sum of the areas of the four crescents is equal to the area of the rectangle.
Find the area of the annulus in terms of the length of the chord which is tangent to the inner circle.
Four identical right angled triangles are drawn on the sides of a square. Two face out, two face in. Why do the four vertices marked with dots lie on one line?
An equilateral triangle is sitting on top of a square. What is the radius of the circle that circumscribes this shape?
Can you find the areas of the trapezia in this sequence?
This shape comprises four semi-circles. What is the relationship between the area of the shaded region and the area of the circle on AB as diameter?
If you know the sizes of the angles marked with coloured dots in this diagram which angles can you find by calculation?
ABCD is a square. P is the midpoint of AB and is joined to C. A line from D perpendicular to PC meets the line at the point Q. Prove AQ = AD.
Find the missing angle between the two secants to the circle when the two angles at the centre subtended by the arcs created by the intersections of the secants and the circle are 50 and 120 degrees.
What can you say about the lengths of the sides of a quadrilateral whose vertices are on a unit circle?
Can you make sense of these three proofs of Pythagoras' Theorem?
Which hexagons tessellate?
Show that among the interior angles of a convex polygon there cannot be more than three acute angles.
Prove Pythagoras' Theorem using enlargements and scale factors.
Points A, B and C are the centres of three circles, each one of which touches the other two. Prove that the perimeter of the triangle ABC is equal to the diameter of the largest circle.
It is obvious that we can fit four circles of diameter 1 unit in a square of side 2 without overlapping. What is the smallest square into which we can fit 3 circles of diameter 1 unit?
Three points A, B and C lie in this order on a line, and P is any point in the plane. Use the Cosine Rule to prove the following statement.
Is the mean of the squares of two numbers greater than, or less than, the square of their means?
Take any rectangle ABCD such that AB > BC. The point P is on AB and Q is on CD. Show that there is exactly one position of P and Q such that APCQ is a rhombus.
Can you make sense of the three methods to work out the area of the kite in the square?
A circle has centre O and angle POR = angle QOR. Construct tangents at P and Q meeting at T. Draw a circle with diameter OT. Do P and Q lie inside, or on, or outside this circle?
A picture is made by joining five small quadrilaterals together to make a large quadrilateral. Is it possible to draw a similar picture if all the small quadrilaterals are cyclic?
Clearly if a, b and c are the lengths of the sides of an equilateral triangle then a^2 + b^2 + c^2 = ab + bc + ca. Is the converse true?
The largest square which fits into a circle is ABCD and EFGH is a square with G and H on the line CD and E and F on the circumference of the circle. Show that AB = 5EF. Similarly the largest. . . .
Here are some examples of 'cons', and see if you can figure out where the trick is.
Prove that the shaded area of the semicircle is equal to the area of the inner circle.
When is it impossible to make number sandwiches?
We have exactly 100 coins. There are five different values of coins. We have decided to buy a piece of computer software for 39.75. We have the correct money, not a penny more, not a penny less! Can. . . .
The diagonal of a square intersects the line joining one of the unused corners to the midpoint of the opposite side. What do you notice about the line segments produced?
L triominoes can fit together to make larger versions of themselves. Is every size possible to make in this way?
What is the largest number of intersection points that a triangle and a quadrilateral can have?
Four jewellers share their stock. Can you work out the relative values of their gems?
These formulae are often quoted, but rarely proved. In this article, we derive the formulae for the volumes of a square-based pyramid and a cone, using relatively simple mathematical concepts.
What fractions can you divide the diagonal of a square into by simple folding?
The diagram shows a regular pentagon with sides of unit length. Find all the angles in the diagram. Prove that the quadrilateral shown in red is a rhombus.
Which set of numbers that add to 10 have the largest product?
Can you discover whether this is a fair game?
Do you know how to find the area of a triangle? You can count the squares. What happens if we turn the triangle on end? Press the button and see. Try counting the number of units in the triangle now. . . .
A huge wheel is rolling past your window. What do you see?
Imagine two identical cylindrical pipes meeting at right angles and think about the shape of the space which belongs to both pipes. Early Chinese mathematicians call this shape the mouhefanggai.
Kyle and his teacher disagree about his test score - who is right?
When number pyramids have a sequence on the bottom layer, some interesting patterns emerge...