1990 AIME Problems/Problem 14
The rectangle below has dimensions and . Diagonals and intersect at . If triangle is cut out and removed, edges and are joined, and the figure is then creased along segments and , we obtain a triangular pyramid, all four of whose faces are isosceles triangles. Find the volume of this pyramid.
To find the volume, we want to use the equation , so we need to find the height of the tetrahedron. By the Pythagorean Theorem, . If we let be the center of a sphere with radius , then lie on the sphere. The cross section of the sphere is a circle, and the center of that circle is the foot of the perpendicular from the center of the sphere. Hence the foot of the height we want to find occurs at the circumcenter of .
From here we just need to perform some brutish calculations. Using the formula ( being the circumradius), we find . By the Pythagorean Theorem,
Finally, we substitute into the volume equation to find .
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