2001 AIME I Problems/Problem 12
Problem
A sphere is inscribed in the tetrahedron whose vertices are 
and 
The radius of the sphere is 
where 
and 
are relatively prime positive integers. Find 
Solution
![[asy]import three; pointpen = black; pathpen = black+linewidth(0.7); currentprojection = perspective(-2,9,4); triple A = (6,0,0), B = (0,4,0), C = (0,0,2), D = (0,0,0); triple E = (2/3,0,0), F = (0,2/3,0), G = (0,0,2/3), L = (0,2/3,2/3), M = (2/3,0,2/3), N = (2/3,2/3,0); triple I = (2/3,2/3,2/3); triple J = (6/7,20/21,26/21); draw(C--A--D--C--B--D--B--A--C); draw(L--F--N--E--M--G--L--I--M--I--N--I--J); label("$I$",I,W); label("$A$",A,S); label("$B$",B,S); label("$C$",C,W*-1); label("$D$",D,W*-1);[/asy]](http://latex.artofproblemsolving.com/b/2/2/b22f739c87d34ee814fc3d229fdade0498040c90.png)
The center 
of the insphere must be located at 
where 
is the sphere's radius.
must also be a distance from the plane 
The signed distance between a plane and a point can be calculated as 
, where G is any point on the plane, and P is a vector perpendicular to ABC.
A vector 
perpendicular to plane can be found as 
Thus 
where the negative comes from the fact that we want to be in the opposite direction of
Finally 
See also
- <url>viewtopic.php?p=384205#384205 Discussion on AoPS</url>
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