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?
Stage 5 - Core Mapping. Cosine rule. Surds. Pythagoras' theorem. Golden ratio. Isosceles triangles. Long problems. Quadratic equations. Mathematical reasoning & proof. Index notation/Indices.
Published June 1998.
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