Stage: 4 and 5 Challenge Level: 
What is the value of $X$ in the equilateral triangle
below?
In how many different ways can you find this out?
Surds. Pythagorean triples. Pythagoras' theorem. Golden ratio. Cosine rule. Long problems. Mathematical reasoning & proof. Quadratic equations. Complex numbers. Isosceles triangles.
Published June 1998.
Copyright © 2007. University of Cambridge. All rights reserved.
NRICH is part of the family of activities in the Millennium Mathematics Project, which also includes the Plus and Motivate sites.
email: nrich@damtp.cam.ac.uk
