Star Prisms & Antiprisms

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(Uniform #78) Heptagrammic 7/3 Prism
Vertices:  14  (14[3])
Faces:9  (7 squares + 2 regular 7/2 heptagrams)
Edges:21
Symmetry:  7-fold Prismatic  (D7h)
Heptagram-Square Angle:  acos(0)    90 degrees
Square-Square Angle:  acos(root[8*(x^3)−4*(x^2)−4*x+1])    
    = acos(cos(π/7))    
180/7 degrees
Dual Solid:  Heptagrammic 7/2 Dipyramid
(values below based on edge length = 1)
Circumscribed Radius:  sqrt(root[448*(x^3)−1232*(x^2)+980*x−239])    
    = sqrt(tan(π/14)*tan(π/14)+2)/2    
≈0.716256777210814081147
Midscribed Radius:  sqrt(root[7*(x^3)−14*(x^2)+7*x−1])    
    = 1/(2*cos(π/14))    
≈0.512858431636276949747
Heptagram Center Radius:  1/2    0.5
Square Center Radius:  sqrt(root[448*(x^3)−560*(x^2)+84*x−1])    
    = tan(π/14)/2    
≈0.11412173719507496904


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