Radius of inscribed and circumscribed sphere in square pyramid.

**Aquameatwad** · June 29th 2011, 06:57 PM

How do you go about finding the radius of a inscribed and Circumscribed sphere inside a pyramid or tetrahedron or basically any 3D object? Any examples or websites that show how its done?

**earboth** · June 29th 2011, 10:26 PM

Originally Posted by Aquameatwad

How do you go about finding the radius of a ... Circumscribed sphere

Take the equation of a sphere with the center $M(m_1, m_2, m_3)$ and the radius r:

$(x-m_1)^2+(y-m_2)^2+(z-m_3)^2=r^2$

You need at least 4 vertices of the solid.
Plug in the coordinates of the vertices into the general equation.
After subtracting one equation from the other three equations (the square values will vanish) you have a system of three linear simultaneous equations in $(m_1, m_2, m_3)$ .
Afterwards you can calculate the length of r.

... or basically any 3D object? ...

That depends on the kind of solid and it would be definitely a very special case.

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