This article is about triangles in which the lengths of the sides and the radii of the inscribed circles are all whole numbers.
A trapezium is divided into four triangles by its diagonals. Suppose the two triangles containing the parallel sides have areas a and b, what is the area of the trapezium?
A farmer has a field which is the shape of a trapezium as illustrated below. To increase his profits he wishes to grow two different crops. To do this he would like to divide the field into two. . . .
If the base of a rectangle is increased by 10% and the area is unchanged, by what percentage (exactly) is the width decreased by ?
Four quadrants are drawn centred at the vertices of a square . Find the area of the central region bounded by the four arcs.
Can you prove this formula for finding the area of a quadrilateral from its diagonals?
Investigate the properties of quadrilaterals which can be drawn with a circle just touching each side and another circle just touching each vertex.
In a right-angled tetrahedron prove that the sum of the squares of the areas of the 3 faces in mutually perpendicular planes equals the square of the area of the sloping face. A generalisation. . . .
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
A finite area inside and infinite skin! You can paint the interior of this fractal with a small tin of paint but you could never get enough paint to paint the edge.
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?
Can you choose your units so that a cube has the same numerical value for it volume, surface area and total edge length?
Manufacturers need to minimise the amount of material used to make their product. What is the best cross-section for a gutter?
Find a quadratic formula which generalises Pick's Theorem.
If I print this page which shape will require the more yellow ink?
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.
Draw three equal line segments in a unit circle to divide the circle into four parts of equal area.
Given a square ABCD of sides 10 cm, and using the corners as centres, construct four quadrants with radius 10 cm each inside the square. The four arcs intersect at P, Q, R and S. Find the. . . .
Three rods of different lengths form three sides of an enclosure with right angles between them. What arrangement maximises the area
Make a poster using equilateral triangles with sides 27, 9, 3 and 1 units assembled as stage 3 of the Von Koch fractal. Investigate areas & lengths when you repeat a process infinitely often.
Six circular discs are packed in different-shaped boxes so that the discs touch their neighbours and the sides of the box. Can you put the boxes in order according to the areas of their bases?
Draw two circles, each of radius 1 unit, so that each circle goes through the centre of the other one. What is the area of the overlap?
Three triangles ABC, CBD and ABD (where D is a point on AC) are all isosceles. Find all the angles. Prove that the ratio of AB to BC is equal to the golden ratio.
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 a general rule for finding the areas of equilateral triangles drawn on an isometric grid?
Triangle ABC is right angled at A and semi circles are drawn on all three sides producing two 'crescents'. Show that the sum of the areas of the two crescents equals the area of triangle ABC.
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. . . .
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?
The square ABCD is split into three triangles by the lines BP and CP. Find the radii of the three inscribed circles to these triangles as P moves on AD.
A and B are two points on a circle centre O. Tangents at A and B cut at C. CO cuts the circle at D. What is the relationship between areas of ADBO, ABO and ACBO?
What is the same and what is different about these circle questions? What connections can you make?
Cut off three right angled isosceles triangles to produce a pentagon. With two lines, cut the pentagon into three parts which can be rearranged into another square.
How efficiently can you pack together disks?
Have a go at creating these images based on circles. What do you notice about the areas of the different sections?
Change the squares in this diagram and spot the property that stays the same for the triangles. Explain...
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?
A white cross is placed symmetrically in a red disc with the central square of side length sqrt 2 and the arms of the cross of length 1 unit. What is the area of the disc still showing?
Can you find the area of a parallelogram defined by two vectors?
In this problem we are faced with an apparently easy area problem, but it has gone horribly wrong! What happened?
A square of area 40 square cms is inscribed in a semicircle. Find the area of the square that could be inscribed in a circle of the same radius.
Which has the greatest area, a circle or a square inscribed in an isosceles, right angle triangle?
Can you draw the height-time chart as this complicated vessel fills with water?
Can you find the areas of the trapezia in this sequence?
Analyse these beautiful biological images and attempt to rank them in size order.
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?
What is the area of the quadrilateral APOQ? Working on the building blocks will give you some insights that may help you to work it out.
Follow the hints and prove Pick's Theorem.
The diagonals of a trapezium divide it into four parts. Can you create a trapezium where three of those parts are equal in area?
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.
Join in this ongoing research. Build squares on the sides of a triangle, join the outer vertices forming hexagons, build further rings of squares and quadrilaterals, investigate.