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'Polycircles' printed from http://nrich.maths.org/
For any regular polygon it is always possible to draw circles with
centres at the vertices of the polygon, and radii equal to half the
length of the edges, to form a "polycircle" in which each circle
just touches its neighbours.
Investigate and explain what happens in the case of non-regular
polygons. Is it always possible to construct three circles with
centres at the vertices of the triangle so that the circles just
Try a numerical example, say the 3 points give a triangle of sides
12, 15 and 13. Find the radii of the circles.
Now use the same method when the sides of the triangle are $a$, $b$
Generalise still more! What about 4 circles? 5 circles? $n$
Experiment with the interactivities below or do the construction
for yourself with ruler and compasses or using dyanamic geometry
The next applet is a 5-sided polygon. Again you can check that
wherever you move the vertices off the polygon, the circles centred
at those points will remain tangent to their two adjacant circles.
Can you calculate the radii given the lengths of the sides of the
In the final applet below we have a quadrilateral. Here you see
that there are 4-sided polygons where the 4 circles are not all
tangent to one another. Can you make the red and green circles lie
on top of one another? Then you have made all the circles tangent
to their neighbours.
Created with GeoGebra
For the final challenge, what is the locus of the centre of
the red and green circles when it moves so that these circles are
coincident, always touching their neighbours, maintaining a perfect
To investigate this problem further, download a copy of
and experiment for
yourself. Geogebra is free educational mathematics software that is
very easy to use and combines dynamic geometry, coordinate
geometry, algebra and calculus. You can also download the
guide for beginners.