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Plane Geometry - Bruce E. Shapiro

Plane Geometry - Bruce E. Shapiro

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Section 28Characterization ofBisectorsTheorem 28.1 (Pointwise Characterization of Angle Bisector) LetA, B, C be non-collinear points and let P be in the interior of ∠BAC. ThenP lies on the angle bisector of ∠BAC if and only if d(P, ←→ AB) = d(P, ←→ AC).Proof. ( ⇒) Assume P is a point on the bisector of ∠BAC (see figure 28.1).Drop perpendiculars from P to ←→ AB and ←→ AC to their respective feet D ∈ ←→ ABand E ∈ ←→ AC.∠P AE = ∠P AD by definition of angle bisection, ∠P EA = ∠P DA becauseboth are right angles, and P A = P A. Hence by AAS, △ADP ∼ = △AEP .Hence DP = P E because they are corresponding sides of congruent trian-Figure 28.1: Theorem 28.1.129

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