As shown in the figure, AE is the bisector of the outer angle of triangle ABC, and angle B is equal to angle C. The formula shows the reason why AE is parallel to BC

Let d be a point on the Ba extension line
∵ AE is the bisector of the outer angle of the triangle ABC
∴∠EAC=1/2 ∠CAD
And ∵ CAD = ∵ B + ∵ C, ∵ B = ∵ C
∴∠EAC=∠C
∴AE‖BC

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