A proof to remember: Double Angle Formulas I (visual proof)

This is a short, animated visual proof of the Double angle identities for sine and cosine. To get the formulas we employ the Law of Sines and the Law of Cosines to an isosceles triangle created by from a reflected copy of a given triangle.

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This animation is the classic visual proof the fact but most likely not the one Euclid used.

For visual proofs of the Law of Sines and Law of Cosines, check out:
   • Law of Sines Visual Proof  
   • Law Of Cosines II (visual proof)   or
   • Law Of Cosines I (visual proof)  

This animation is based on a visual proof by Sidney H. Kung from the March 1996 issue of The College Mathematics Journal (https://www.jstor.org/stable/2687407 - page 155).

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