Star Dipyramids & Trapezohedra

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Heptagrammic 7/3 Dipyramid
Vertices:  9  (7[4] + 2[7])
Faces:14  (isosceles triangles)
Edges:21  (14 short + 7 long)
Symmetry:  7-fold Prismatic  (D7h)
Dihedral Angle:  acos(−root[239*(x^3)−227*(x^2)+45*x−1])    
    = acos(−(97−96*cos(π/7)−4/cos(π/7))/239)    
≈91.454683627 degrees
Dual Solid:  Heptagrammic 7/3 Prism
(values below based on unit-edge-length Heptagrammic 7/3 Prism)
Short Edge (14):  root[7*(x^3)−28*(x^2)+64]    
    = 8*sin(2*π/7)/sqrt(7)    
≈2.3640361940244141018
Long Edge (7):  root[(x^3)−4*(x^2)−4*x+8]    
    = 1/sin(π/14)    
≈4.4939592074349341221
[7]-Vertex Radius (2):  root[7*(x^3)−28*(x^2)+28*x−8]    
    = 1/(2*cos(π/14)*cos(π/14))    
≈0.52604754180084351532
[4]-Vertex Radius (7):  sqrt(root[7*(x^3)−56*(x^2)+112*x−64])    
    = 1/sin(π/7)    
≈2.3047648709624865052
Edge-scribed Radius:  sqrt(root[7*(x^3)−14*(x^2)+7*x−1])    
    = 1/(2*cos(π/14))    
≈0.512858431636276949747
Inscribed Radius:  sqrt(root[1673*(x^3)−2520*(x^2)+784*x−64])    
    = sqrt((1140−872*cos(π/7)−116/cos(π/7))/1673)    
≈0.36721994020729052507