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Weekly Challenge 21: Trig Identity

Stage: 4 and 5 Short Challenge Level: Challenge Level:1
See all short problems arranged by curriculum topic in the short problems collection

In this diagram $O$ is the centre of a unit circle. Use it to find a trigonometrical relationship between $A$ and $2A$.
Did you know ... ?

Trigonometrical expressions can often be derived algebraically or geometrically. In more advanced applications the functions of trigonometry are defined as infinite power series rather than geometric ratios.