Star Prisms & Antiprisms

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(Uniform #80) Heptagrammic Crossed Antiprism
Vertices:  14  (14[4])
Faces:16  (14 equilateral triangles + 2 regular 7/3 heptagrams)
Edges:28
Symmetry:  7-fold Prismatic  (D7h)
Heptagram-Triangle Angle:  acos(root[27*(x^3)−45*(x^2)−3*x+13])    
    = acos((4*sin(π/14)+1)/3)    
≈50.947818141 degrees
Triangle-Triangle Angle:  acos(sqrt(root[27*(x^3)−189*(x^2)+105*x−7]))    
    = acos(tan(2*π/7)/sqrt(3))    
≈316.3835916349 degrees
Dual Solid:  Heptagrammic Concave Trapezohedron
(values below based on edge length = 1)
Circumscribed Radius:  sqrt(root[448*(x^3)−560*(x^2)+224*x−29])    
    = sqrt(cot(2*π/7)*cot(2*π/7)+5)/4    
≈0.59350462329578707220
Midscribed Radius:  sqrt(root[448*(x^3)−224*(x^2)+28*x−1])    
    = 1/(4*sin(2*π/7))    
≈0.31976200192248315144
Heptagram Center Radius:  sqrt(root[64*(x^3)+48*(x^2)−16*x+1])    
    = sqrt(cos(π/7)*(1−cos(π/7)))    
≈−0.29870381144714635357
Triangle Center Radius:  −sqrt(root[12096*(x^3)−3024*(x^2)+1])    
    = −sqrt((3*cot(2*π/7)*cot(2*π/7)−1)/48)    
≈0.13752964967649988638


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