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Math Help - How to calculate a circle's radius when tangent to 2 intersecting lines

  1. #1
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    How to calculate a circle's radius when tangent to 2 intersecting lines

    In the diagram below, circle c is tangent to lines p, q, and r. Lines q and s are perpendicular to each other. Angle a is 20 degrees (the subtended angle is 40 degrees). What is the radius of circle c, and how did you calculate it? Thanks.

    Involute

    Last edited by Involute; March 24th 2010 at 01:49 PM. Reason: Changed reference to diagram.
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  2. #2
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    If I understand the diagram correctly, this is true.
    \sin(20^o)=\frac{r}{1-r}.
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  3. #3
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    Looks right to me. Thanks!
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