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Plane Geometry - Bruce E. Shapiro

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Section 52Arc LengthIn this section we begin our consideration of geometry on the surface of asphere. We first need to figure out what we mean by a “line” on the surfaceof a sphere. Since we would like out geometry to satisfy as m much ofneutral geometry as possible (we will find that it is not possible to satsifyall the postulates of neutral geometry in spherical geometry), we will beginwith the result that the distance between two points is the length of a linecontaining those two points. By the triangle inequality, we know that wecan not construct any shorter path between those two points by any pairof line segments. Applying this concept repeatedly, any path composed ofline segments will have an overall length at least as long as the length of asingle segment.Figure 52.1: The segment AB is the shortest path between A and B inneutral geometry.305

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