The sine of an angle is equal to the cosine of its complement. Can you explain why and does this rule extend beyond angles of 90 degrees?

A dot starts at the point (1,0) and turns anticlockwise. Can you estimate the height of the dot after it has turned through 45 degrees? Can you calculate its height?

An environment that simulates a protractor carrying a right- angled triangle of unit hypotenuse.

Draw three equal line segments in a unit circle to divide the circle into four parts of equal area.

If you were to set the X weight to 2 what do you think the angle might be?

On a nine-point pegboard a band is stretched over 4 pegs in a "figure of 8" arrangement. How many different "figure of 8" arrangements can be made ?

Join some regular octahedra, face touching face and one vertex of each meeting at a point. How many octahedra can you fit around this point?

An observer is on top of a lighthouse. How far from the foot of the lighthouse is the horizon that the observer can see?

The area of a square inscribed in a circle with a unit radius is, satisfyingly, 2. What is the area of a regular hexagon inscribed in a circle with a unit radius?

A belt of thin wire, length L, binds together two cylindrical welding rods, whose radii are R and r, by passing all the way around them both. Find L in terms of R and r.

There are many different methods to solve this geometrical problem - how many can you find?

This problem in geometry has been solved in no less than EIGHT ways by a pair of students. How would you solve it? How many of their solutions can you follow? How are they the same or different?. . . .

In this problem we are faced with an apparently easy area problem, but it has gone horribly wrong! What happened?

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.

One side of a triangle is divided into segments of length a and b by the inscribed circle, with radius r. Prove that the area is: abr(a+b)/ab-r^2

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

Ten squares form regular rings either with adjacent or opposite vertices touching. Calculate the inner and outer radii of the rings that surround the squares.

Can you deduce the familiar properties of the sine and cosine functions starting from these three different mathematical representations?

How would you design the tiering of seats in a stadium so that all spectators have a good view?

Trigonometry, circles and triangles combine in this short challenge.

How can you represent the curvature of a cylinder on a flat piece of paper?

The third of three articles on the History of Trigonometry.

The second of three articles on the History of Trigonometry.

The first of three articles on the History of Trigonometry. This takes us from the Egyptians to early work on trigonometry in China.

What is an AC voltage? How much power does an AC power source supply?

Can you explain what is happening and account for the values being displayed?

Two perpendicular lines lie across each other and the end points are joined to form a quadrilateral. Eight ratios are defined, three are given but five need to be found.

Two places are diametrically opposite each other on the same line of latitude. Compare the distances between them travelling along the line of latitude and travelling over the nearest pole.

The Earth is further from the Sun than Venus, but how much further? Twice as far? Ten times?

Prove Pythagoras' Theorem for right-angled spherical triangles.

How far should the roof overhang to shade windows from the mid-day sun?

Use simple trigonometry to calculate the distance along the flight path from London to Sydney.

Make a clinometer and use it to help you estimate the heights of tall objects.

Logo helps us to understand gradients of lines and why Muggles Magic is not magic but mathematics. See the problem Muggles magic.

What angle is needed for a ball to do a circuit of the billiard table and then pass through its original position?

The length AM can be calculated using trigonometry in two different ways. Create this pair of equivalent calculations for different peg boards, notice a general result, and account for it.

A moveable screen slides along a mirrored corridor towards a centrally placed light source. A ray of light from that source is directed towards a wall of the corridor, which it strikes at 45 degrees. . . .

What is the longest stick that can be carried horizontally along a narrow corridor and around a right-angled bend?

Describe how to construct three circles which have areas in the ratio 1:2:3.

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 squares are drawn on the sides of a triangle ABC. Their areas are respectively 18 000, 20 000 and 26 000 square centimetres. If the outer vertices of the squares are joined, three more. . . .

What are the shortest distances between the centres of opposite faces of a regular solid dodecahedron on the surface and through the middle of the dodecahedron?

The sides of a triangle are 25, 39 and 40 units of length. Find the diameter of the circumscribed circle.

Two problems about infinite processes where smaller and smaller steps are taken and you have to discover what happens in the limit.

Follow instructions to fold sheets of A4 paper into pentagons and assemble them to form a dodecahedron. Calculate the error in the angle of the not perfectly regular pentagons you make.

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.

From the measurements and the clue given find the area of the square that is not covered by the triangle and the circle.

Follow the instructions and you can take a rectangle, cut it into 4 pieces, discard two small triangles, put together the remaining two pieces and end up with a rectangle the same size. Try it!

Prove that the shaded area of the semicircle is equal to the area of the inner circle.

Straight lines are drawn from each corner of a square to the mid points of the opposite sides. Express the area of the octagon that is formed at the centre as a fraction of the area of the square.