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Your simplest math question of the day---but I really want to know!

Is there a name for the half-diagonal of a rectangle?

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    $\begingroup$ Half diagonal of the rectangle? [No, in earnest: Why invent special names if a short, simple description is available?] $\endgroup$ – vonbrand Feb 26 '13 at 20:51
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    $\begingroup$ "radius of the circumscribed circle"? $\endgroup$ – user7530 Feb 26 '13 at 21:02
  • $\begingroup$ @vonbrand--agreed, simple is best and half-diagonal is just fine with me. I'm using this measurement in collision tests that I'm working on, and ironically (for me) "circumradius" conjurs in my mind an outward-looking measurement where half-diagonal conjurs and internal measurement. If it were a different word, I might think otherwise. How about that--"feelings" in math! $\endgroup$ – markE Feb 26 '13 at 22:08
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For regular polygons (i.e. if this were a square) this would be called the "radius" or "circumradius". Likewise for regular polygons, the distance from the center to a center of a side is called the "apothem" or "inradius".

Circumradius is an appropriate name for rectangles as well, because it is possible to inscribe any rectangle in a circle, and the segment you describe is a radius of that circle.

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