Radical Center: What Is It About?
A Mathematical Droodle


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What if applet does not run?

Explanation

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Copyright © 1996-2018 Alexander Bogomolny

Radical Center

The diagram in the applet can be viewed as presenting several problems. Far as I can see, all are related to the notion of Radical Center of three circles. Let me know if you see additional interpretations.


This applet requires Sun's Java VM 2 which your browser may perceive as a popup. Which it is not. If you want to see the applet work, visit Sun's website at https://www.java.com/en/download/index.jsp, download and install Java VM and enjoy the applet.


What if applet does not run?
  1. The radical axes of three circles taken by two meet in a point (radical center of the three circles.) The applet presents a particular case where two of the circles are tangent while all three have common points. Indeed, the radical axis of tangent circles is their common tangent. The radical axis of two intersecting circles is the line containing their common chord.

  2. Two circles with centers O1 and O2 are tangent at point T. Through a point P on their common tangent draw two lines PS1 and PS1. Assume this creates the intersection points A1, B1, and A2, B2, respectively. Then the four points A1, B1, B2, A1 are cyclic.

  3. The applet may also be seen as presenting a solution to a construction problem: Find a circle through two given points A1, B1 tangent to a given circle (that with center O2.)

    One solution is this. Draw any circle through the given points A1, B1 and intersecting the given circle. Let this be circle O. The radical axes of the circles O, O2, and the unknown circle O1 meet at some point P. The tangent from P to the circle O2 serves as the radical axis of O2 and the unknown circle O1. Therefore, the point of tangency T belongs to the two circles, O1 in particular. Now the three points A1, B1, and T define O1 uniquely.

References

  1. S. Savchev, T. Andreescu, Mathematical Miniatures, MAA, 2003, pp. 112-113

Radical Axis and Radical Center

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Copyright © 1996-2018 Alexander Bogomolny

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