Other tags that relate to Handy Angles
Circumference and arc length. Mathematical reasoning & proof. Area - circles, sectors and segments. 2D shapes and their properties. Area - squares and rectangles. Visualising. Creating and manipulating expressions and formulae. Regular polygons and circles. Circle properties and circle theorems. Pythagoras' theorem.There are 161 results
Broad Topics > Thinking Mathematically > Mathematical reasoning & proof
The Pillar of Chios
Age 14 to 16
Challenge Level
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.
Coins on a Plate
Age 11 to 14
Challenge Level
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.
Encircling
Age 14 to 16
Challenge Level
An equilateral triangle is sitting on top of a square. What is the radius of the circle that circumscribes this shape?
Salinon
Age 14 to 16
Challenge Level
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?
Round and Round
Age 14 to 16
Challenge Level
Prove that the shaded area of the semicircle is equal to the area of the inner circle.
A Chordingly
Age 11 to 14
Challenge Level
Find the area of the annulus in terms of the length of the chord which is tangent to the inner circle.
Circle Box
Age 14 to 16
Challenge Level
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?
Zig Zag
Age 14 to 16
Challenge Level
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?
Towering Trapeziums
Age 14 to 16
Challenge Level
Can you find the areas of the trapezia in this sequence?
Pythagorean Triples I
Age 11 to 16
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!
Fitting In
Age 14 to 16
Challenge Level
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. . . .
Pythagorean Triples II
Age 11 to 16
This is the second article on right-angled triangles whose edge lengths are whole numbers.
Cyclic Quad Jigsaw
Age 14 to 16
Challenge Level
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?
Long Short
Age 14 to 16
Challenge Level
What can you say about the lengths of the sides of a quadrilateral whose vertices are on a unit circle?
Three Balls
Age 14 to 16
Challenge Level
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?
Similarly So
Age 14 to 16
Challenge Level
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.
Pareq Exists
Age 14 to 16
Challenge Level
Prove that, given any three parallel lines, an equilateral triangle always exists with one vertex on each of the three lines.
No Right Angle Here
Age 14 to 16
Challenge Level
Prove that the internal angle bisectors of a triangle will never be perpendicular to each other.
Ratty
Age 11 to 14
Challenge Level
If you know the sizes of the angles marked with coloured dots in this diagram which angles can you find by calculation?
Lens Angle
Age 14 to 16
Challenge Level
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.
Rhombus in Rectangle
Age 14 to 16
Challenge Level
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.
Cyclic Quadrilaterals
Age 11 to 16
Challenge Level
Draw some quadrilaterals on a 9-point circle and work out the angles. Is there a theorem?
Square Mean
Age 14 to 16
Challenge Level
Is the mean of the squares of two numbers greater than, or less than, the square of their means?
Pythagoras Proofs
Age 14 to 16
Challenge Level
Can you make sense of these three proofs of Pythagoras' Theorem?
Folding Fractions
Age 14 to 16
Challenge Level
What fractions can you divide the diagonal of a square into by simple folding?
Cosines Rule
Age 14 to 16
Challenge Level
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.
Folding Squares
Age 14 to 16
Challenge Level
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?
Chameleons
Age 11 to 14
Challenge Level
Whenever two chameleons of different colours meet they change colour to the third colour. Describe the shortest sequence of meetings in which all the chameleons change to green if you start with 12. . . .
Con Tricks
Age 11 to 14
Here are some examples of 'cons', and see if you can figure out where the trick is.
Mouhefanggai
Age 14 to 16
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.
Find the Fake
Age 14 to 16
Challenge Level
There are 12 identical looking coins, one of which is a fake. The counterfeit coin is of a different weight to the rest. What is the minimum number of weighings needed to locate the fake coin?
Volume of a Pyramid and a Cone
Age 11 to 14
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.
Kite in a Square
Age 14 to 16
Challenge Level
Can you make sense of the three methods to work out the area of the kite in the square?
Converse
Age 14 to 16
Challenge Level
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?
Pent
Age 14 to 18
Challenge Level
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.
Matter of Scale
Age 14 to 16
Challenge Level
Prove Pythagoras' Theorem using enlargements and scale factors.
Triangular Intersection
Age 14 to 16 Short
Challenge Level
What is the largest number of intersection points that a triangle and a quadrilateral can have?
Parallel Universe
Age 14 to 16
Challenge Level
An equilateral triangle is constructed on BC. A line QD is drawn, where Q is the midpoint of AC. Prove that AB // QD.
Convex Polygons
Age 11 to 14
Challenge Level
Show that among the interior angles of a convex polygon there cannot be more than three acute angles.
Go Forth and Generalise
Age 11 to 14
Spotting patterns can be an important first step - explaining why it is appropriate to generalise is the next step, and often the most interesting and important.
The Frieze Tree
Age 11 to 16
Patterns that repeat in a line are strangely interesting. How many types are there and how do you tell one type from another?
Cross-country Race
Age 14 to 16
Challenge Level
Eight children enter the autumn cross-country race at school. How many possible ways could they come in at first, second and third places?
Logic
Age 7 to 14
What does logic mean to us and is that different to mathematical logic? We will explore these questions in this article.
Disappearing Square
Age 11 to 14
Challenge Level
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. . . .
Concrete Wheel
Age 11 to 14
Challenge Level
A huge wheel is rolling past your window. What do you see?
Picture Story
Age 14 to 16
Challenge Level
Can you see how this picture illustrates the formula for the sum of the first six cube numbers?
Natural Sum
Age 14 to 16
Challenge Level
The picture illustrates the sum 1 + 2 + 3 + 4 = (4 x 5)/2. Prove the general formula for the sum of the first n natural numbers and the formula for the sum of the cubes of the first n natural. . . .
Gift of Gems
Age 14 to 16
Challenge Level
Four jewellers share their stock. Can you work out the relative values of their gems?
NRICH is part of the family of activities in the Millennium Mathematics Project.