Without using a calculator, computer or tables find the exact values of cos36cos72 and also cos36 - cos72.
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.
ABCDE is a regular pentagon of side length one unit. BC produced meets ED produced at F. Show that triangle CDF is congruent to triangle EDB. Find the length of BE.
Explore the geometry of these dart and kite shapes!
Nick Lord says "This problem encapsulates for me the best features of the NRICH collection."
Leonardo who?! Well, Leonardo is better known as Fibonacci and this article will tell you some of fascinating things about his famous sequence.
Find the link between a sequence of continued fractions and the ratio of succesive Fibonacci numbers.
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.
Explain how to construct a regular pentagon accurately using a straight edge and compass.
A voyage of discovery through a sequence of challenges exploring properties of the Golden Ratio and Fibonacci numbers.
When is a Fibonacci sequence also a geometric sequence? When the ratio of successive terms is the golden ratio!
You add 1 to the golden ratio to get its square. How do you find higher powers?
Rectangle PQRS has X and Y on the edges. Triangles PQY, YRX and XSP have equal areas. Prove X and Y divide the sides of PQRS in the golden ratio.
Draw whirling squares and see how Fibonacci sequences and golden rectangles are connected.
Solve an equation involving the Golden Ratio phi where the unknown occurs as a power of phi.
Find a connection between the shape of a special ellipse and an infinite string of nested square roots.
Draw a square and an arc of a circle and construct the Golden rectangle. Find the value of the Golden Ratio.
What is the relationship between the arithmetic, geometric and harmonic means of two numbers, the sides of a right angled triangle and the Golden Ratio?
An iterative method for finding the value of the Golden Ratio with explanations of how this involves the ratios of Fibonacci numbers and continued fractions.
Show that the arithmetic mean, geometric mean and harmonic mean of a and b can be the lengths of the sides of a right-angles triangle if and only if a = bx^3, where x is the Golden Ratio.