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Plane Geometry - Bruce E. Shapiro

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SECTION 31. QUADRILATERALS IN NEUTRAL GEOMETRY 151Figure 31.6: The shaded triangles are congruent, hence the correspondingdiagonals of the Saccheri Quadrilateral are congruent (theorem 31.15).DCDCABABTheorem 31.16 The summit angles of a Saccheri Quadrilateral are congruent.Proof. Repeat the argument in the previous proof, but with the upper-halftriangles. The triangles are congruent by SSS - they share the same top;the diagonals are congruent; and the sides are congruent. Hence the cornerangles are congruent.Theorem 31.17 Let □ABCD be a Saccheri quadrilateral. Then the segmentjoining the midpoints of the base and summit is perpendicular to thebase and summit.Proof. Let M be the bisector of AB and N the bisector of CD (see figure31.7).Figure 31.7: Proof of theorem 31.17. The line segment connecting the midpointsof the base and summit of a Saccheri Quadrilateral is perpendicularto both the base and the summit.Revised: 18 Nov 2012 « CC BY-NC-ND 3.0.

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