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Plane Geometry - Bruce E. Shapiro

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208 SECTION 39. CIRCLESTheorem 39.16 Let Γ = C(A, r) be a circle. Choose any point B ∈ Γ andany point C not in Γ. Define the functionbyf(α) : (0, 180) ↦→ [0, ∞)f(α) = CD(x)where D(x) ∈ H C,←→ ABis chosen so that µ(∠DAB) = α (see figure 39.7).Then f(x) is continuous.Figure 39.7: The function f(α) : (0, 180) ↦→ [0, ∞) is continuous.Proof. Let α ∈ (0, 180).By the triangle inequality,HenceCD(x) < CD(a) + D(a)D(x)CD(a) < CD(x) + D(a)D(x)By the definition of f(x),Taking limits,CD(a) − D(a)D(x) < CD(x)< CD(a) + D(a)D(x)f(a) − D(a)D(x) < f(x) < f(a) + D(a)D(x)lim (f(a) − D(a)D(x)) < lim f(x)x→α x→α< lim (f(a) + D(a)D(x))x→α« CC BY-NC-ND 3.0. Revised: 18 Nov 2012

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