Proofs without words: Ptolemy's theorem and cosine's law

Ptolemy's theorem

Derrick W. & Hirstein J. (2012). Proof Without Words: Ptolemy’s Theorem, The College Mathematics Journal, 43 (5) 386-386. DOI: 10.4169/college.math.j.43.5.386 (via Cut the Knot)

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Law of cosines

Kung S.H. (1990). Proof without Words: The Law of Cosines, Mathematics Magazine, 63 (5) 342. DOI: 10.2307/2690911 (pdf)

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Law of cosines via Ptolemy's theorem

Kung S.H. (1992). Proof without Words: The Law of Cosines via Ptolemy's Theorem, Mathematics Magazine, 65 (2) 103. DOI: 10.2307/2690488 (pdf)