Maths Olympiad Prep

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, 2003

Geometry Difficulty 3.6 AMC 10/12 Find the answer Italy

Problem:

In a rectangular parallelepiped PP the length of the diagonal is 133\sqrt{133} and the total surface area is 228228. Knowing that one of the sides is a mean proportional between the other two, the volume of PP is

Pick one

Solution

Solution:

The answer is (D). If we denote by x,yx, y and zz the lengths of the sides in increasing order, the hypotheses translate as
{xy+xz+yz=2282=114x2+y2+z2=133xz=y2 \left\{ \begin{array}{l} x y + x z + y z = \frac{228}{2} = 114 \\ x^{2} + y^{2} + z^{2} = 133 \\ x z = y^{2} \end{array} \right.

From the first two we obtain (x+y+z)2=(x2+y2+z2)+2(xy+xz+yz)=361(x + y + z)^{2} = \left(x^{2} + y^{2} + z^{2}\right) + 2(x y + x z + y z) = 361 and hence x+y+z=19x + y + z = 19. Substituting the third into the first we obtain y(x+y+z)=xy+xz+yz=114y(x + y + z) = x y + x z + y z = 114, from which y=6y = 6. Therefore the volume is xyz=y3=216x y z = y^{3} = 216. On the other hand, a parallelepiped with sides x=4,y=6,z=9x = 4, y = 6, z = 9 satisfies the hypotheses.

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from it; metadata (topic, difficulty) added by this project.