let AABC be a triangle S². Let the dual point C'ES² of C be defined by the following three conditions: (i) d(C¹, A)=π/2 (ii) d(C¹, B)=π/2 (iii) d(C¹, C)<π/2 A' and B' are defined analogously. Thus we get a dual triangle AA' B' C'. More precisely, AABC is a non- degenerate triangle, and it follows (you may assume) that AA' B' C' is too. let AABC be a triangle S^2. Let the dual point C'ES^2 of C be defined by the following three conditions: (i) d(C¹, A)=π/2 (ii) d(C¹, B)=π/2 (iii) d(C¹, C)<π/2 A' and B' are defined analogously. Thus we get a dual triangle AA' B' C'. More precisely, AABC is a non- degenerate triangle, and it follows (you may assume) that AA' B' C' is too. (a) Show that the conditions (i)-(iii) determine a unique point C' by, in terms of vectors in R³ thinking. (For example, (i) is in terms of perpendicularity to interpret.) (b) Show that d(C¹, X) = π/2 for all points X on the line (in the sentence of S², so great circle) line(A, B). (c) Show that the dual triangle of the dual triangle is the original one triangle is (AA" B" C"=AABC) by proving that C"=C. (The other cases, A" = A and B" = B, are of course analogous.)
let △ABC be a triangle S2 (the two-dimensional sphere). Let the dual point C'∈S2 of C be defined by the following three conditions:
(i) d(C' , A)=π/2
(ii) d(C' , B)=π/2
(iii) d(C' , C)≤π/2
A' and B' are defined analogously. Thus we get a dual triangle △A' B' C'. More precisely, △ABC is a non-degenerate triangle, and it follows (you may assume) that △A' B' C' is too.
(a) Show that the conditions (i)–(iii) determine a unique point C' by, in terms of
thinking. (For example, (i) is in terms of perpendicularity to interpret.)
(b) Show that d(C' , X) = π/2 for all points X on the line (in the sentence of S2, so great circle) line(A, B).
(c) Show that the dual triangle of the dual triangle is the original one
triangle is (△A'' B'' C''=△ABC) by proving that C''=C. (The other
cases, A'' = A and B'' = B, are of course analogous.)
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