Triangle has vertices A = ( 0 , 0 ) , B = ( 9 , 0 ) , C = ( 3 , 6 ) . Its Steiner inellipse is dilated from triangle centroid. New ellipse trisects all three sides of the triangle A B C .
Given that the dilation factor can be expressed as b a , where a and b are positive integers with b square-free.
Find a + b .
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Nice problem! I love problems that require very little computation when "done right".
May I steal this problem for the upcoming sixth edition of my text "Linear Algebra with Applications"? (with citation, of course)
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Reminder: Send me the amazon link when it's published.
Yes, you can use it.
Fine. Very simple.
Equation for big ellipse 4 x 2 + 2 x y + 7 y 2 − 3 6 x − 3 6 y + 7 2 = 0 and
a 2 = 1 1 − 1 3 7 2
b 2 = 1 1 + 1 3 7 2
c b 2 + b 2 = a 2
c b = 3 2 1 3
From https://brilliant.org/problems/complex-ellipse/?ref_id=1595309 find for little ellipse c l = 1 3
c l c b = 3 2
Answer 2 + 3 = 5 .
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It is a factor of 3 2 . It is the same as a factor between trisecting circle and incircle radii for an equilateral triangle. It is independent of a triangle, in other words, Steiner inellipse dilated by a factor of 3 2 from centroid will trisect sides of any triangle.