10.07.2015 Views

Plane Geometry - Bruce E. Shapiro

Plane Geometry - Bruce E. Shapiro

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Section 49Triangles in Hyperbolic<strong>Geometry</strong>Theorem 49.1 For every ɛ > 0 there exists an isosceles triangle △BP Cwith σ(△BP C) < ɛ and δ(△BP C) > 180 − ɛ.Proof. Let ɛ > 0 be given.Since lim x→∞ κ(x) = 0 there exists some y such that ∀a > y, κ(a) < ɛ/4.Choose one such a.Let △P AB be a triangle with right angle at A and both legs of length a.Since −→−→P B intersects AB then µ(∠AP B) < K(a).Since ∠ABP = ∠AP B,andσ△BP C = µ(∠ACP ) + µ(∠ABP )+ 2µ(∠AP B)= 4µ(∠AP B)< 4κ(a)< ɛδ(△BP C) = 180 − σ(△BP C)< 180 − ɛTheorem 49.2 For every ɛ > 0 there is a right triangle △ABC such thatσ(△ABC) > 180 − ɛ and δ(△ABC) < ɛ.283

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