# Geometry Quiz for radius of a circle



## Bikeworks (Oct 24, 2012)

HELP... Maybe there is a better way to figure this, but, see the attached photo first for a quick view of what I am trying to do...

I need to figure out the radius of a circle so that I can draw an accurate arch on a piece of trim. I plan to draw the arch on a piece of 1/4" mdf and then use this as a template to route a nice smooth curve on my lower rail of a blanket chest I am making for my daughter. Any help or suggestions would be great.

My geometry skills are rusty. I know that you can extend the line CD to the center of the circle E. Then with imaginary triangles you measure one angle and with the length of one leg and that angle (degrees) you can figure length D to E and add C to D to get the radius.

Line AB is 26"
Line CD is .75"
What is length CE (CD + DE)?

Thanks,
Tim


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## Cassandra (Mar 15, 2006)

Find the radius from a chord - OnlineConversion Forums

r = (0.75*0.75 + 26*26/4)/(2*0.75) = 113.04


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## Bikeworks (Oct 24, 2012)

Brilliant, thanks


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## jw2170 (Jan 24, 2008)

Tim, can you just use a flexible rule and the 3 pin method to draw your arch to make a template?


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## JCJCJC (May 15, 2012)

Bisect AB with a compass. Draw another line similar to AB somwhere else on the circle and bisect that too. Where the two bisecting lines meet is the centre. Do a third one for the hell of it and hope it intersects the same point.


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## Knothead47 (Feb 10, 2010)

Online conversion is great. I have it saved as a favorite.


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## bigegg (Aug 20, 2011)

JCJCJC said:


> Bisect AB with a compass. Draw another line similar to AB somwhere else on the circle and bisect that too. Where the two bisecting lines meet is the centre. Do a third one for the hell of it and hope it intersects the same point.


Why not bisect BC and AC?


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## AxlMyk (Jun 13, 2006)

Cassandra said:


> Find the radius from a chord - OnlineConversion Forums
> 
> r = (0.75*0.75 + 26*26/4)/(2*0.75) = 113.04


Can you believe that thread is 9 pages long?


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