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Interior angle bisector theorem

Let ABC be a triangle. Let AM be the bisector of the angle A. Then we have:



Let d be the line parallel at the line AM that passes through C. Let F be the intersection between this line d and AB.

AFC = BAM (because AM is parallel with FC)
BAC = MAC (because AM is the bisector of BAC)
MAC = ACF (because AM is parallel with FC)

Because AFC = ACF the triangle AFC is isoscele. So AF = AC.

From the Thales theorem we have:


because AF = AC we have:

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