Dipyramids & Trapezohedra

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Heptagonal Dipyramid
Vertices:  9  (7[4] + 2[7])
Faces:14  (isosceles triangles)
Edges:21  (7 short + 14 long)
Symmetry:  7-fold Prismatic  (D7h)
Dihedral Angle:  acos(-root[239*(x^3)−227*(x^2)+45*x−1])    ≈133.089524326 degrees
Dual Solid:  Heptagonal Prism
(values below based on unit-edge-length Heptagonal Prism)
Short Edge (7):  root[(x^3)−4*(x^2)−4*x+8] = 1/cos(π/7)    ≈1.1099162641747423828
Long Edge (14):  root[7*(x^3)−28*(x^2)+64] = 3−tan(π/14)^2    ≈2.9479049163983129694
[4]-Vertex Radius (7):  sqrt(root[7*(x^3)−56*(x^2)+112*x−64])    
    = 1/sin(2*π/7)    
≈1.2790480076899326057
[7]-Vertex Radius (2):  root[7*(x^3)−28*(x^2)+28*x−8]    
    = 1/(2*(sin(π/7)^2))    
≈2.6559705552113635356
Edge-scribed Radius:  sqrt(root[7*(x^3)−14*(x^2)+7*x−1])    
    = 1/(2*sin(π/7))    
≈1.1523824354812432526
Inscribed Radius:  sqrt(root[1673*(x^3)−2520*(x^2)+784*x−64])    
    = 2*sqrt(1673*(436*(cos(π/7)^2)    
    +116*cos(π/7)+9))/1673    
≈1.0571625914335797820
Volume:sqrt(root[729*(x^3)−54432*(x^2)    
    +564480*x−1404928])    
    = 2*sqrt(7)/(3*sin(π/14))    
≈7.9265989766278435864