# Trigonometry by Watching

### See it

### Do it yourself

Created with GeoGebra

### Explanation

Hopefully, the demonstration above made it pretty obvious that the purpose of this page is to prove the trigonometric identity

$\arctan(1)+\arctan(2)+\arctan(3)=\pi.$

That identity has been established elsewhere from several viewpoints and also by paper folding.

The present demonstration (which is due to **John Molokach**) appears to be the most effortless. The demonstration is artfully using the properties of the famous 3-4-5 (Egyptian) triangle.

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