Official Solution:A rectangular solid, \(x\) by \(y\) by \(z\), is inscribed in a sphere, so that all eight of its vertices are on the sphere. If \(x\), \(y\) and \(z\) are positive integers, and the radius of the sphere is \(\frac{\sqrt{14}}{2}\), what is the volume of the rectangular solid? A. \(6\)
B. \(11\)
C. \(12\)
D. \(18\)
E. \(22\)
The radius of \(\frac{\sqrt{14}}{2}\) means that the diameter of the sphere is \(\sqrt{14}\).
Notice that the diameter of the sphere is equal to the long diagonal of the rectangular solid, which is equal to \(\sqrt{x^2+y^2+z^2}\). So, we have that \(\sqrt{x^2+y^2+z^2}=\sqrt{14}\).
Square: \(x^2+y^2+z^2=14\)
Since \(x\), \(y\) and \(z\) are positive integers, then \((x, y, z)=(1, 2, 3)\) (in any order).
The volume of the rectangular solid is therefore \(xyz=6\).
Answer: A