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Proof without Words: A Sum and Product of Three Tangents

by Roger B. Nelsen (Lewis and Clark College)

This article originally appeared in:
Mathematics Magazine
February, 2002

Subject classification(s): Geometry and Topology | Trigonometry | Trigonometric Identities
Applicable Course(s): 2.2 Trigonometry | 4.1 Introduction to Proofs | 4.9 Geometry

The author presents a visual proof that the sum of the tangents of the angles in an acute triangle is equal to the product of the tangents.

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