# How to describe foсi of en ellipse inscribed in the triangle thru triangles angles points?

I was looking at Marden's theorem and could not help but wonder how foсi of en ellipse inscribed in the triangle can be described thru triangles angles points?

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What do you mean by "triangles angles points"? –  Michael Hardy Dec 31 '11 at 18:16

Suppose the three vertices are $z_1,z_2,z_3$, and those are complex numbers.
Let $p(z)=(z-z_1)(z-z_2)(z-z_3)$ be the polynomial whose roots are those vertices. Marden's theorem says the foci of the Steiner inellipse are the zeros of \begin{align} p'(z) & = \frac{d}{dz}(z-z_1)(z-z_2)(z-z_3) \\ \\ & = \frac{d}{dz}(z^3-z^2(z_1+z_2+z_3)+z(z_1z_2+z_1z_3+z_2z_3)-z_1z_2z_3) \\ \\ & = 3z^2- 2z(z_1+z_2+z_3) + (z_1z_2+z_1z_3+z_2z_3). \end{align} Setting this equal to $0$ and solving the quadratic equation, we get $$\frac{z_1+z_2 + z_3}{3} \pm \frac13\sqrt{z_1^2+z_2^2+z_3^2-z_1z_2-z_1z_3-z_2z_3}.$$ The term before the "$\pm$" is the average of the three vertices, so that's the center of the ellipse.