Angles, Polygons and Geometrical Proof (age 14-16)
Cyclic Quadrilaterals
Draw some quadrilaterals on a 9-point circle and work out the angles. Is there a theorem?
Triangle Midpoints
You are only given the three midpoints of the sides of a triangle. How can you construct the original triangle?
Circles in Quadrilaterals
Explore when it is possible to construct a circle which just touches all four sides of a quadrilateral.
Nicely Similar
If the hypotenuse (base) length is 100cm and if an extra line splits the base into 36cm and 64cm parts, what were the side lengths for the original right-angled triangle?
Two Ladders
Two ladders are propped up against facing walls. The end of the first ladder is 10 metres above the foot of the first wall. The end of the second ladder is 5 metres above the foot of the second wall. At what height do the ladders cross?
Sitting Pretty
A circle of radius r touches two sides of a right angled triangle, sides x and y, and has its centre on the hypotenuse. Can you prove the formula linking x, y and r?
Kite in a Square
Can you make sense of the three methods to work out what fraction of the total area is shaded?
Trapezium Four
The diagonals of a trapezium divide it into four parts. Can you create a trapezium where three of those parts are equal in area?
Partly Circles
What is the same and what is different about these circle questions? What connections can you make?
Napkin
A napkin is folded so that a corner coincides with the midpoint of an opposite edge . Investigate the three triangles formed .
Angle Trisection
It is impossible to trisect an angle using only ruler and compasses but it can be done using a carpenter's square.
Squirty
Using a ruler, pencil and compasses only, it is possible to construct a square inside any triangle so that all four vertices touch the sides of the triangle.
Same Length
Construct two equilateral triangles on a straight line. There are two lengths that look the same - can you prove it?
Angles, Polygons and Geometrical Proof Short Problems
A collection of short problems on Angles, Polygons and Geometrical Proof.
Isosceles Seven
Is it possible to find the angles in this rather special isosceles triangle?
The Square Under the Hypotenuse
Can you work out the side length of a square that just touches the hypotenuse of a right angled triangle?
Quad in Quad
Join the midpoints of a quadrilateral to get a new quadrilateral. What is special about it?
You may also be interested in this collection of activities from the STEM Learning website, that complement the NRICH activities above.