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How do you find the center of two concentric circles with just a straightedge? (lemmy.sdf.org)

submitted 3 weeks ago by CanadaPlus@lemmy.sdf.org to c/math@lemmy.sdf.org

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The Wikipedia article on Steiner constructions mentions it, but doesn't explain it, and the source linked is a book I don't have. This has come up in a practical project.

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[–] badcommandorfilename@lemmy.world 2 points 3 weeks ago (1 children)

Scribe a tangent to either circle. A line perpendicular to the tangent that passes through the intersection of the tangent must pass through the centre. Do it twice and where those two lines intersect is the centre point.

Not sure if that will work for your practical application - getting it truly perpendicular might be hard.

[–] CanadaPlus@lemmy.sdf.org 1 points 3 weeks ago

Exactly. I could do a straightedge construction of 90 degrees, but not until I have a center, so I'm back at square one.