Steiner inellipse

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In geometry, the Steiner inellipse of a triangle, named after Jakob Steiner, is the unique ellipse inscribed in the triangle and tangent to the sides at their midpoints. By comparison, the inscribed circle of a triangle is tangent to the sides, but not necessarily at the midpoints.

According to Marden's theorem, if the three vertices of the triangle are the complex zeros of a cubic polynomial, then the foci of the Steiner inellipse are the zeroes of the derivative of the polynomial.