# Circumcircle of Three Parabola Tangents

24 February 2015, Created with GeoGebra

The circumcircle of a triangle formed by three tangents to a parabola, passes through the focus of the parabola.

This is known as Lambert's theorem.

### Proof

Let the tangent at $C$ intersect tangents $AS$ and $BS$ in points $U$ and $V,$ respectively. Theorem of similar triangles, applied twice, gives

$\angle FSU = \angle FBS = \angle FVU,$

which tells us that the quadrilateral $SUFS$ is cyclic.

### x-Axis is simson of the focus

As we saw earlier, $x$-axis is the pedal curve of the parabola with respect to its focus. In other words, $x$-axis consists of the feet of the perpendiculars from the focus to the tangents to the parabola. This means that $x$-axis is the simson of the focus with respect to the circumcircle of any three tangents to the parabola [Honsberger, p. 48].

### Parabola from four tangents

Lambert's theorem suggests a construction of parabola from four tangents. Any three tangents determine a circle that passes through $F.$ Two such circles determine F uniquely. Reflections of $F$ in any two tangents produce two points on the directrix.

### References

1. H. Dörrie, 100 Great Problems Of Elementary Mathematics, Dover Publications, NY, 1965
2. R. Honsberger, Episodes in Nineteenth and Twentieth Century Euclidean Geometry, MAA, 1995.

### Simson Line - the simson

• Simson Line: Introduction
• Simson Line
• Three Concurrent Circles
• 9-point Circle as a locus of concurrency
• Miquel's Point
• Angle Bisector in Parallelogram
• Simsons and 9-Point Circles in Cyclic Quadrilateral
• Reflections of a Point on the Circumcircle
• Simsons of Diametrically Opposite Points
• Simson Line From Isogonal Perspective
• Pentagon in a Semicircle
• Simson Line in Disguise
• Two Simsons in a Triangle
• Carnot's Theorem
• A Generalization of Simson Line

Copyright © 1996-2018 Alexander Bogomolny

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