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Plane Geometry - Bruce E. Shapiro

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SECTION 49. TRIANGLES IN HYPERBOLIC GEOMETRY 285Figure 49.2: Left:Any triangle can be divided into two right triangles. IfD is not between A and B then relabel the vertices to use a different edge.Right: △A ′ EC ′ ∼ = △ADC if A ′ E = AC and AD = C ′ E. This is possiblebecause the shortest side of △EF G has length d and all the sides of △ABChave length less than d.Since AB = AD + BD then BD < d and AD < d.By the scalene inequality CD < AC < d.As shown in figure 49.2, we can embed a triangle that is congruent to eachof △BDC and △ADC in △EF G.By additivity of defecthenceδ(△BDC) < δ(△EF G) < ɛ/2δ(△ADC) < δ(△EF G) < ɛ/2δ(△ABC) = δ(△BDC) + δ(△ADC)< ɛ 2 + ɛ 2 = ɛTheorem 49.4 For every pair of points A and B and for every ɛ > 0 thereexists a d > 0 such that if C ∉ ←→ AB and AC < d then δ(△ABC) < ɛ.Proof. Let ɛ > 0 be given.Choose a point P such that P ∗ A ∗ B, and choose D such thatµ(∠BP D) = 90 − ɛRevised: 18 Nov 2012 « CC BY-NC-ND 3.0.

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