Teacher2Teacher 
Q&A #6322 
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From: Loyd <Loydlin@aol.com> To: Teacher2Teacher Public Discussion Date: 2012010511:50:22 Subject: Re: Finding the radius of a circle when you only have an arc On 2004030820:02:12, Philip Wetherington wrote: >I have a board that is 6 inches wide and 20 inches long. I want to cut >an arc in the board that starts at the lower left bottom corner of the >board and runs to the lower right corner of the board. In the middle >of the board the height of the arc will be 4 inches from the bottom of >the board (midpoint along the bottom of the board). If I know the >formula for this, I can mark the point 4 inches up from the midpoint >at the bottom of the board, run down the radius line the "proper >distance",(this is the unknown I'm looking for), anchor the end of a >string, run back up the distance of the radius, wrap the string around >a pencil and draw my arc. >This is not a hypothetical question. I am a woodworker and find myself >having to construct such an arc quite often using a time consuming >"trial and error" method. A formula using the 22 inches and 4 inches >to find the center of the circle, and thus the radius would be a >lifesaver. > >Thank you for your help, > >Philip R. Wetherington >Macon, GA >philva@mindspring.com > There is an answer or two already posted but they require the radius to solve the problem. So, I found a theorem that should help get the radius of the circle that contains the arc. "If two chords intersect in a circle then the products of the segments of the chords are equal." So, the diameter of the circle and the 22 inch chord are both chords that intersect. The diameter is 4 + S and the other chord is 22 inches divided into segments of 11 inches each. So we multiply: 4xS = 11x11 S= 121/4 so the Diameter of the circle is 4 + 121/4. Half of that is the radius. I found a similar problem in a high school geometry book under a chapter called special segments in a circle were a clock maker was trying to repair a broken escape wheel in an antique clock. The book reference is:
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