Tangent Circles
By: Lacy Gainey
Given 2 circles, A and B, we can find a third circle, C, that is tangent to both of these circles.
Lets start off by constructing two circles.
Next, place a point on circle A. Label this point K.
Construct a line through the center of circle A and point K. Label this line L.
At point K construct a circle with the same radius as circle B. Construct a point at the intersection of circle K and line L. Label the point M.
Construct a line segment from point M to the center of circle B. Find the midpoint of this segment and label it N.
Construct a line though point N that is perpendicular to the line segment BM.
Construct a point at the intersection of the perpendicular line and line L. Label this point O.
Lastly, highlight point O and point k and construct a circle using the center + point command.
Circle O is tangent to both circle A and circle B.
Click here for a GSP script tool for the construction of this tangent circle.
Lets look at constructing a tangent circle when circle A and circle B have different characteristics.
Case 1: When circle B is lies completely inside of circle A.
This case is represented in the construction we did above.
When circle B is completely inside of circle A, the tangent circle O is inside of circle A as well.
Case 2: When circle A and circle B overlap.
When circle A and circle B overlap, the tangent circle O is still contained within circle A.
Case 3: When circle A and circle B are disjoint.
When the two circles are disjoint, circle A is contained within the tangent circle O.
