A pair of solutions to the Apollonius three-circle problem.  The point <i>R</i> is the radical center of the three given circles, <i>s1</i>, <i>s2</i> and <i>s3</i>.  The dashed circle <i>I</i> intersects all of these given circles at right angles (orthogonally); hence, inversion in this transforms these given circles into themselves.  The circles <i>b1</i> and <i>b2</i> are the solutions to Apollonius' problem; the same inversion transforms <i>b1</i> into <i>b2</i>, and vice versa.
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