Let ABC be an equilateral triangle with K its circumcircle.
Let P be a point on K (different from A, B, and C) and m
the tangent line to K at point P.
Let D, E, and F be points on m such that AD, BE, and CF
are perpendicular to m.
Prove that AD + BE + CF equals twice the length of
an altitude of triangle ABC.
Center the circle at the origin and let its radius be r.
The altitude is 3r/2 so twice this is 3r.
Each of A,B,C forms an angle with the positive x axis. Call the smallest angle x, then the others are x+120 and x120.
WLOG let point P be (0,r) and the tangent line be y=r
The three lengths are r sin(x) + r, r sin(x+120) + r, and r sin(x120) + r
The sum is 3r + r [sin(x+120) + sin(x) + sin(x120)]
So now to show sin(x+120) + sin(x) + sin(x120) = 0
sin (x+120) = sin(x)cos(120) + cos(x)sin(120) = .5sin(x)
sin (x120) = sin(x)cos(120)  cos(x)sin(120) = .5sin(x)
sin (x) = sin(x)
So the sum of these terms is zero and the sum of the three lengths is 3r.
Edited on June 11, 2010, 4:01 pm

Posted by Jer
on 20100611 16:01:23 