Hirano's Construction of a Regular Pentagon

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This Demonstration shows Yosifusa Hirano's construction of a regular pentagon with a ruler and compass.

Contributed by: S. Eugene Stewart and Kenneth E. Caviness (September 2014)
Open content licensed under CC BY-NC-SA



Proof that the constructed figure is a pentagon:

Let circle be a unit circle with center with and , since is the midpoint of . Then by the Pythagorean theorem is . Since is , we have . and angle is an inscribed angle that subtends a semicircle, thus angle is a right angle. Letting be angle , , and thus . Then by the central angles theorem, angle , making angle , which proves that is a pentagon.


[1] A. Bogomolny. "Regular Pentagon Construction by Y. Hirano." Interactive Mathematics Miscellany and Puzzles. (Sept 8, 2014) www.cut-the-knot.org/pythagoras/PentagonConstruction.shtml.

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