Ford's Touching Circles

by Rita Snyder, Matt Sorrells, and B.J. Jackson


The challenge is to construct three circles tangent to each other and to a given line.

At first this seems impossible until we look at some equations. First we find two points on the given line a/b and c/d such that a+d = b+c. To help illustrate this we will use the points where a=1, b=2, c=2, and d=3.

 

 


Now we want to find a point such that (a+c)/(b+d)so for our example our third point will be 3/5. In order to locate this point we must cut the segment between 1/2 and 2/3 into 5 equal parts.

To do this we will first construct an arbitrary line through point A.

We then construct a circle with center A

Locate where the circle intersects the arbitrary line and construct another circle with the center as the point of intersection and the radius equal to that of our first circle.

Continue this construction process until you have identified five points of intersection on our arbitrary line.

Now construct a line through our fifth point of intersection and our point B.

Construct lines through our other four points parallel to our new line and identify the points where our parallel lines intersect our given line

Now we can identify our point 3/5


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Next we want to construct circles through points A and B tangent to the line such that if we say the points A and B equaled p/q, the diameter of the circle would equal . In order to get the correct ratio we need to solve = x/30 (the common denominator of 1/2, 3/5, and 2/3). So for our example we need to solve 1/4 = x/30 thus 4x = 30, so x=15/2 or 7.5 to construct the circle through point A.

Once we have determined the value of x, we then construct our circle such that it is tangent to our given line. In order to ensure tangency, we construct a line perpendicular to our given line through point A.

Then construct a circle with center A and radius equal to 1/5 of our given segment and identify the point where the circle intersects our perpendicular line. Now construct another circle with the center as the point of intersection and radius same a previous circle.

Continue this process until there are 8 points of intersection.

Then find the midpoint between the 7th and 8th point of intersection. The distance from this midpoint and point A is equal to 15/2 or 7.5, thus it is the diameter of your circle tangent to the given line.

If you find the midpoint of this diameter you can construct your first Ford Circle.

Use this same process to construct the circle through point B that is also tangent to the given line. The same process can be used to construct the circle through our point labeled 3/5.

To verify that our constructions are correct we'll use the construction method for constructing a circle tangent to a given circle and a given line.

We will use our three Ford's circles and the given line to reconstruct tangent circles to our circles and given line see if they appear in the same location. We'll recolor the circles to make them more identifiable.

First lets start with our red circlewe want to construct a circle tangent to the red circle given our line and we want the new circle to intersect the point 2/3 on our given line.

The first thing we do is construct a line perpendicular to our given line through point 2/3, then construct a circle with the center at point 2/3 and the radius equal to that of our red circle.

Now label the points where the new circle intersects the perpendicular line point J and K. Then construct a segment from the center of our red circle to point K and construct a perpendicular bisector to this segment.

The point where our perpendicular bisector crosses our first perpendicular line is the center of our new circle, if you notice it is exactly where the center of the blue circle is located.

We'll now use the same contruction method with our blue circle being the given circle and our given line, and using the point 3/5 for a new tangent circle.

Again you see that where our perpendicular bisector crosses our first perpendicular line is exactly where the center of the green circle is located.

Thus our first method of constructing our Ford's Touching Circles is accurate.


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