The Laws of Sines and Cosines
by Alexander Bogomolny · Cut The Knot
Reference page collecting classical proofs of both laws: Euclid's Elements II.12 treatment of the obtuse case, two proofs without words from Nelsen using power of a point and Ptolemy's theorem, Maor's projection argument, and dissection proofs.
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mathsisfun.com
MathIsFun.com is a beginner-friendly math site that explains concepts with clear, step-by-step lessons and interactive activities. It covers topics from algebra and linear equations to geometry, fractions, data, and more, with practice problems, worksheets, and quizzes.