In geometry, given a triangle ABC, there exist unique points A´, B´, and C´ on the sides BC, CA, AB respectively, such that:[1] This is point X369 in Clark Kimberling's Encyclopedia of Triangle Centers.
[2] Uniqueness and a formula for the trilinear coordinates of X369 were shown by Peter Yff late in the twentieth century.
The formula involves the unique real root of a cubic equation.
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