sentences:  The disphenoids are the tetrahedra in which all four faces have the same perimeter, the tetrahedra in which all four faces have the same area, and the tetrahedra in which the angular defects of all four vertices equal .They are the polyhedra having a net in the shape of an acute triangle, divided into four similar triangles by segments connecting the edge midpoints.The volume of a disphenoid with opposite edges of length "l", "m" and "n" is given by
  
  The circumscribed sphere has radius (the circumradius)
  
  and the inscribed sphere has radius
  
  where "V" is the volume of the disphenoid and "T" is the area of any face, which is given by Heron's formula.