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Inscribed sphere
Sphere tangent to every face of a polyhedron / From Wikipedia, the free encyclopedia
In geometry, the inscribed sphere or insphere of a convex polyhedron is a sphere that is contained within the polyhedron and tangent to each of the polyhedron's faces. It is the largest sphere that is contained wholly within the polyhedron, and is dual to the dual polyhedron's circumsphere.
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The radius of the sphere inscribed in a polyhedron P is called the inradius of P.