Star Dipyramids & Trapezohedra

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Heptagrammic 7/2 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(−(41−16*cos(π/7)+28/cos(π/7))/239)    
≈103.961168876 degrees
Dual Solid:  Heptagrammic 7/2 Prism
(values below based on unit-edge-length Heptagrammic 7/2 Prism)
Short Edge (14):  root[7*(x^3)+28*(x^2)−64]    
    = cot(π/7)*cot(π/7)−3    
≈1.3119411104227270712
Long Edge (7):  root[(x^3)+4*(x^2)−4*x−8]    
    = 4*cos(π/7)−2    
≈1.6038754716096765049
[7]-Vertex Radius (2):  root[7*(x^3)−28*(x^2)+28*x−8]    
    = 1/(2*sin(2*π/7)*sin(2*π/7))    
≈0.81798190298779294908
[4]-Vertex Radius (7):  sqrt(root[7*(x^3)−56*(x^2)+112*x−64])    
    = 1/cos(π/14)    
≈1.02571686327255389949
Edge-scribed Radius:  sqrt(root[7*(x^3)−14*(x^2)+7*x−1])    
    = 1/(2*sin(2*π/7))    
≈0.63952400384496630287
Inscribed Radius:  sqrt(root[1673*(x^3)−2520*(x^2)+784*x−64])    
    = sqrt((472−464*cos(π/7)+334/cos(π/7))/1673)    
≈0.50381834168200655316