# A Geometric Proof of Machin's Formula

by Roger B. Nelsen (Lewis and Clark College)

This article originally appeared in:
Mathematics Magazine
December, 1990

Subject classification(s): Algebra and Number Theory | Number Theory | Famous Problems | Geometry and Topology | Geometric Proof
Applicable Course(s): 3.2 Mainstream Calculus II | 4.1 Introduction to Proofs | 4.8 History of Math

The author presents a geometric proof of Machin's formula, which expresses $\frac {\pi}{4}$ as the difference of two inverse tangent functions.

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