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 Trigonometry in extended hyperbolic space and extended de Sitter space Bull. Korean Math. Soc. 2009 Vol. 46, No. 6, 1099-1133 https://doi.org/10.4134/BKMS.2009.46.6.1099Published online November 1, 2009 Yunhi Cho University of Seoul Abstract : We study the hyperbolic cosine and sine laws in the extended hyperbolic space which contains hyperbolic space as a subset and is an analytic continuation of the hyperbolic space. And we also study the spherical cosine and sine laws in the extended de Sitter space which contains de Sitter space $S^n_1$ as a subset and is also an analytic continuation of de Sitter space. In fact, the extended hyperbolic space and extended de Sitter space are the same space only differ by $-1$ multiple in the metric. Hence these two extended spaces clearly show and apparently explain that why many corresponding formulas in hyperbolic and spherical space are very similar each other. From these extended trigonometry laws, we can give a coherent and geometrically simple explanation for the various relations between the lengths and angles of hyperbolic polygons, and relations on de Sitter polygons which lie on $S^2_1$, and tangent laws for various polyhedra. Keywords : hyperbolic space, volume, analytic continuatio MSC numbers : 51M10, 51M25, 53B30 Downloads: Full-text PDF