Why excircles?

Geometry Level 5

Let \(I_1,I_2,I_3\) be the centres of the excircles of \(\Delta ABC\). Denote the area of \(\Delta ABC\) by \(\Delta_O\) and the area of \(\Delta I_1I_2I_3\) by \(\Delta_I\).

Define the set \(E\) as follows: \[ E=\left\{k\in \mathbb{R}\:|\:\Delta_I\geq k\Delta_O\: \text{for all triangles}\:\Delta ABC\right\}\]

Enter the value of \(\sup \{E\}\) up to three decimal places.


  • \(\sup\) denotes the supremum of a set.
  • \(\mathbb{R}\) denotes the set of all real numbers.

This problem is part of my set: Geometry


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