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Ratio of Circle Radii that Share tangent lines

I don't know how to explain the arrangement, so I've attached an image. Forgive my notation, I'm not used to writing math problems.

The points that are black are known. The points that are red are unknown. and are tangent to BOTH circles. is tangent only to circle 2 (the larger circle).

Conceptually, I think this is easy. The center of circle 2 ( ) lies on , and the radius is such that it is tangent to both and . Without any other conditions, there would be an infinite number of solutions. However, there is an additional line that bounds circle 2. So, unless I am mistaken, there is now only one solution.

However, I don't know what I'm doing wrong. I've tried using similar triangles, but because neither nor the radius for circle 2 is known, I haven't been able to determine the ratio of the two triangles. In fact, everything I've tried seems to require a system of equations.

So I tried the following, where is the distance formula between points and :

However, when I write the distance formulas for three dimensions, I end up with nine unknowns; three for each of the following points, , , and .

So, I tried an additional three equations based on the distance of the center of circle 2 to each of the bounding lines using the following formula:

I needed three more equations, so I tried parameterizing the equation for , yielding:

where

However, this led to a binomial equation in x, y and z. So, instead I tried using the equation for each of the three bounding lines, since the respective tangent point is a solution to the corresponding line equation. Unfortunately, I'm not sure if I'm not solving the system of equations correctly, or if my equations are not independent from each other.

Again, I feel like there should be an elegant solution to this. Perhaps something similar to finding the tangent points of a circle inscribed inside a triangle.

Any help would be greatly appreciated.

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I'm not sure I understand your post

Quote:

Originally Posted by

**sa-ri-ga-ma** Find the equation of the tangent P1P2.

Write it in the form y = mx + c.

In this case, and have the same y-coordinates, so,

Quote:

Condition for line to be a tangent to a circle C2 is

find r2. T11 is the radius of C1. Now find the ratio of r1 and r2.

When using the above equation, I get a result that is not the radius of circle 2 (See Attached Image). Also note that Circle 3 does not pass through , and if it did, it would only be a coincidence.