Tangent Circles

Alex Szatkowski

Given two circles we can find two different tangent circles.

To begin, we need two circles, Circle A and Circle B.

Let P be a point on the circumferance of circle A. This will be the point of tangency of our new triangle. Next, construct a circle C with center P and the same radius as circle B. Construct a line k that passes through the center of both circle A and circle C.

Next, label the point of intersection of line k and circle C, I. From our new point I to the center of circle B create a line segment and label it j.

Next we will find the midpoint of line j and label it M and construct a line through M that is perpindicular to line j labeled line l. The point Q is where line l and k intersect.

Lastly, we will construct circle Q with point Q as the center and radius length QP. Circle Q is now tangent to the two given circles, circle A and circle B.

Here you will find a gsp tool for this tangent circle.

As I said before, there are two tangent circles. The other tangent circle contains circle B on the inside.


Let's look at when the given circles A and B have different characteristics.

1. When circle B is completely inside circle A

When circle B is completly inside circle A, the tangent (purple) circle is inside circle A as well.

2. When circle A and circle B overlap.

When the two given circles overlap, the tangent circle (color purple) in still inside circle A.

3. When the two given circles are disjoint.

When circle A and B are disjoint, circle A is enclosed in the tangent circle (purple)