Tangential Circles Write-up


Jim Wilson




Two larger circles with equal radii pass through each other's centers. A smaller circle can be created inside the overlapping region so that it is tangent to the other circles. (Tangent means that the circles touch each other but do not cross over each other, nor do they leave any gaps.) Compare the area and circumference of the smaller circle to the area and circumference of the larger circle.


First, let’s construct a GSP sketch for the problem.   Take a segment AB as the radius of one of the large circles and construct the two large circles.

Construct the internal circle.    We know that if the circle inside is tangent at the center points, then the radius of the small circle is one half of the radius of the large circle. 
So construct the segment connecting the two centers and take its midpoint.   That is the center of the small circle.


Click here to open a GSP file of the discussion.


Take measures of the two circumferences and the two areas; compute their ratios.


Let   r   be the radius of the small circle and   R   the radius of the large circle.   Then   R = 2r.



The ratio of the areas is


The Ratio of the circumferences is