# The Fourth Harmonic Point of a Triangle

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The fourth harmonic point of a triangle is an invariant point under a certain geometric transformation. Given a triangle with and fixed, extend the segment to another fixed point . Draw a line through to intersect the line at and the line at . Let be the intersection of the lines and . Let the line intersect at . Then , called the fourth harmonic point, is invariant either by moving or changing the slope of the line .

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Contributed by: Shenghui Yang (June 2012)

Open content licensed under CC BY-NC-SA

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References

[1] J. Gray, *Worlds Out of Nothing: A Course in the History of Geometry in the 19th Century*, London: Springer-Verlag, 2007.

[2] L. Cremona,* Elements of Projective Geometry*, 3rd ed. (C. Leudesdorf, trans.), New York: Dover, 1960.

[3] R. A. Johnson, *Modern Geometry*, New York: Houghton–Mifflin, 1929.

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