Hi guys I'm having trouble determining the equation to find an unknown radius of a circle that is tangent to 2 lines, and the minimum distance from the intersection of the 2 lines to the circumference of the circle is known (Intersection of the bisector line to the circumference of the circle).

I attached a picture to show the information i have for the problem, the basic problem is, R=??? based on D and A?: Diagram of Problem

  • $\begingroup$ @amd I am not certain that the OP considers that many cases: this question hopefully arose from a metrology problem (how to measure the diameter of a sphere using a conical gage, or something like that). $\endgroup$
    – Jean Marie
    May 18, 2017 at 6:41
  • $\begingroup$ Is it a metrology problem ? $\endgroup$
    – Jean Marie
    May 18, 2017 at 6:51

1 Answer 1


enter image description here

Let $I$ be the point of intersection of the tangents.

Let T be the point of tangency of one of these tangents.

Let $O$ be the center of the circle.

Let $B:=\tfrac{A}{2}$.

(I am indebted to @amd who has "short circuited" a first unduly long proof):

In right triangle OTI,

$$\sin(B)=\dfrac{OT}{OI} \ \ \ \iff \ \ \ \sin(B)=\dfrac{R}{R+D}$$

which we can invert under the following form:

$$R=D \dfrac{\sin(B)}{1-\sin(B)} \ \ \ \iff \ \ \ R=D \dfrac{\sin(A/2)}{1-\sin(A/2)}$$

There is also a proof using the power of a point (here $I$) but it is more complicated.(https://en.wikipedia.org/wiki/Power_of_a_point));

Remark: I think that this question arose from a metrology problem. I am right ?


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