Tetrahedron incenter and circumcenter

Geometry Level pending

Just like triangles, any tetrahedron has an incenter which is the center of the unique inscribed sphere that touches its four faces, and a circumcenter which is the center of the unique sphere that passes through its four vertices. Given the tetrahedron with vertices A = ( 10 , 0 , 0 ) , B = ( 5 , 12 , 0 ) , C = ( 5 , 2 , 0 ) , D = ( 0 , 0 , 10 ) A = (10, 0, 0), B = (5, 12, 0) , C = (-5, 2, 0) , D = (0, 0, 10) find the distance between its incenter and its circumcenter.


The answer is 1.0832.

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1 solution

Mark Hennings
Dec 7, 2020

As I showed for the previous question , we can calculate the incentre of the tetrahedron to have coordinates ( 5 233 + 10 249 5 313 17 + 233 + 249 + 313 , 12 233 + 2 313 17 + 233 + 249 + 313 , 170 17 + 233 + 249 + 313 ) \left(\frac{5\sqrt{233} + 10\sqrt{249} - 5\sqrt{313}}{17 + \sqrt{233}+ \sqrt{249} + \sqrt{313}}\,,\,\frac{12\sqrt{233}+2\sqrt{313}}{17 + \sqrt{233}+ \sqrt{249} + \sqrt{313}}\,,\,\frac{170}{17 + \sqrt{233}+ \sqrt{249} + \sqrt{313}}\right) The outcentre of the tetrahedron is the point of intersection of the three planes with equations r ( a b ) = 1 2 ( a 2 b 2 ) r ( a c ) = 1 2 ( a 2 c 2 ) r ( a d ) = 1 2 ( a 2 d 2 ) \begin{aligned} \mathbf{r} \cdot (\mathbf{a} - \mathbf{b}) & = \; \tfrac12(|\mathbf{a}|^2 -|\mathbf{b}|^2) \\ \mathbf{r} \cdot (\mathbf{a} - \mathbf{c}) & = \; \tfrac12(|\mathbf{a}|^2 -|\mathbf{c}|^2) \\ \mathbf{r} \cdot (\mathbf{a} - \mathbf{d}) & = \; \tfrac12(|\mathbf{a}|^2 -|\mathbf{d}|^2) \end{aligned} and hence has coordinates ( 99 34 , 139 34 , 99 34 ) \big(\tfrac{99}{34},\tfrac{139}{34},\tfrac{99}{34}\big) Thus the distance between the outcentre and the incentre is 1.083176362 \boxed{1.083176362} .

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