Calculate a Radius From a Straightedge and One Gap Measurement
Lay a straightedge across the curve and measure the gap at its center. Radius = chord squared, divided by eight times the gap, plus half the gap — no center point needed.
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Tips
Check Whether a Framing Square Is Actually Square With a Compass and Straightedge
Draw a straight line, swing a semicircle off it with a compass, and pick any two points on the arc plus the line's endpoint — the angle between them is always exactly 90 degrees, by a 2,600-year-old geometry rule. Lay your square's corner against that guaranteed right angle to see how far off it really is.
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