pentagon

[David Ash]

Let O be the center of a circle with radius OA. Construct a perpendicular to OA at O which meets the circle at B. Let C be the midpoint of OB. Let the angle bisector of angle OCA meet OA at D. Construct a perpendicular to OA at D which meets the circle at E.

Prove that AE is one side of a regular pentagon with center at O.

[t...@bellefleurbooks.com]

Dear David:

I agree. There is a simple proof, using trigonometry. Good problem.

Sincerely,

Tom

On 2021-09-28 20:26, 'David Ash' via Competition Corner Participant

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[vincent miller]

I smell cos 36-cos 72

Sent from my iPhone

> On Oct 2, 2021, at 6:50 PM, t...@bellefleurbooks.com wrote:
>
> Dear David:

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