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'Weekly Challenge 21: Trig Identity' printed from http://nrich.maths.org/
||In this diagram $O$ is the centre of a unit circle. Use it to
find a trigonometrical relationship between $A$ and $2A$.
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