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WikiPedia: Định lý Ptoleme http://hypertextbook.com/eworld/chords.shtml http://www.mathalino.com/reviewer/derivation-formu... http://www.mathpages.com/home/kmath099/kmath099.ht... http://demonstrations.wolfram.com/PtolemysTheorem/ http://mathworld.wolfram.com/PtolemyInequality.htm... http://aleph0.clarku.edu/~djoyce/java/elements/Euc... http://aleph0.clarku.edu/~djoyce/java/elements/boo... http://ads.harvard.edu/books/1543droc.book/ http://www.atara.net/deep_secrets/index.html http://www.cut-the-knot.org/proofs/ptolemy.shtml