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Plane Geometry - Bruce E. Shapiro

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260 SECTION 45. EUCLIDEAN CONSTRUCTIONSFigure 45.10: Construction of a tangent line to a circle, drawn through apoint that is outside the circle (construction 45.9).Figure 45.11: Construction of a rational number (construction ).SegmentUV has length q/p.Theorem 45.18 (Mohr-Mascheroni Theorem) All constructions thatcan be performed with the straight-edge and compass can be performedwith the compass alone.Theorem 45.19 (Paper Folding Theorem) All constructions that canbe performed with a straight-edge and compass can be performed by foldingand creasing paper.The following result tells us that the following regular polygons are constructible:those with 3, 4, 5, 6, 8, 10, 12, 15, 16, 17, 20, 24 sides, amongTheorem 45.20 (Polygon Construction, Gauss) A regular polygoncan be inscribed in a circle by means of a straightedge and a compass ifand only if the number of sides can be written asn = 2 x p 1 p 2 ...p k« CC BY-NC-ND 3.0. Revised: 18 Nov 2012

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