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'Angle Trisection' printed from http://nrich.maths.org/

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A classical Greek problem was to find a way to trisect an angle using just a ruler and a pair of compasses. However, this is impossible.

It is possible though, to trisect an angle using a carpenter's square, as demonstrated by the interactivity below.

Can you explain why this works?

Can you extend the idea to trisect an obtuse angle?

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