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Plane Geometry - Bruce E. Shapiro

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SECTION 30. TRIANGLES IN NEUTRAL GEOMETRY 143Figure 30.6: Additivity of defect for triangles.By definition 30.9,δ(△ECA) = 180 − (β + ɛ + ζ)δ(△ABE) = 180 − (η + γ + δ)henceδ(△ECA) + δ(△ABE)= 360 − (β + ɛ + ζ) − (η + γ + δ)= 360 − (ζ + (ɛ + δ) + η + (β + γ))By the angle addition postulate, α = β + γ.Since δ and ɛ form a linear pair, ɛ + δ = 180.Thusδ(△ECA) + δ(△ABE)= 360 − (ζ + 180 + η + α)= 180 − (ζ + η + α)= δ(△ABC)(2) Alternate proof. By theorem 30.3,σ(△ACE) + σ(△ABE) =σ(△ABC) + 180(180 − δ(△ACE))+(180 − δ(△ABE)) =(180 − δ(△ABC)) + 180δ(△ACE)δ(△ABE) =δ(△ABC)Revised: 18 Nov 2012 « CC BY-NC-ND 3.0.

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