Star Prisms & Antiprisms

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(Uniform #79) Heptagrammic 7/3 Antiprism
Vertices:  14  (14[4])
Faces:16  (14 equilateral triangles + 2 regular 7/3 heptagrams)
Edges:28
Symmetry:  7-fold Antiprismatic  (D7v)
Heptagram-Triangle Angle:  acos(−sqrt(root[189*(x^3)−315*(x^2)+63*x−1]))    
    = acos(−cot(2*π/7)/sqrt(3))    
≈117.4143079054 degrees
Triangle-Triangle Angle:  acos(root[27*(x^3)−9*(x^2)−27*x+1])    
    = acos((1−4*sin(π/14))/3)    
≈87.900284033 degrees
Dual Solid:  Heptagrammic 7/3 Trapezohedron
(values below based on edge length = 1)
Circumscribed Radius:  sqrt(root[64*(x^3)−144*(x^2)+80*x−13])    
    = sqrt(tan(2*π/7)*tan(2*π/7)+5)/4    
≈0.64091811725597805808
Midscribed Radius:  root[8*(x^3)+8*(x^2)−2*x−1]    
    = cos(π/7)−1/2    
≈0.40096886790241912624
Heptagram Center Radius:  sqrt(root[448*(x^3)−112*(x^2)+1])    
    = sqrt(3−cot(2*π/7)*cot(2*π/7))/4    
≈0.38438556440965090093
Triangle Center Radius:  sqrt(root[1728*(x^3)−2160*(x^2)+144*x+1])    
    = sqrt((3*tan(2*π/7)*tan(2*π/7)−1)/48)    
≈0.27828528472345480038


References:[1]Jean Paul Albert Badoureau, Mémoire sur les Figures Isocèles,
Journal de l'École polytechnique 49 (1881), 47-172.