Problem

Problem

Let OO be a point inside a ΔABC\Delta ABC , such that ∠OAB=∠OBC=∠OCA=w\angle OAB = \angle OBC = \angle OCA = w . Then, show that cot⁡w=cot⁡A+cot⁡B+cot⁡C\cot w = \cot A + \cot B + \cot C .
Intuitive explanation

The key is recognizing that point OO is the first isogonic center (or first Brocard point), where equal angles are "swept" from each vertex to the opposite side. Rather than chasing angles directly, use the area method: express the total area of △ABC\triangle ABC as a sum of three smaller triangles formed by OO, then relate those areas to the cotangents of the angles. The constraint that all three angles equal ww creates a special relationship that forces cot⁡w\cot w to equal the sum of the triangle's angle cotangents.

Intuitive explanation

The key is recognizing that point OO is the first isogonic center (or first Brocard point), where equal angles are "swept" from each vertex to the opposite side. Rather than chasing angles directly, use the area method: express the total area of △ABC\triangle ABC as a sum of three smaller triangles formed by OO, then relate those areas to the cotangents of the angles. The constraint that all three angles equal ww creates a special relationship that forces cot⁡w\cot w to equal the sum of the triangle's angle cotangents.

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