Sphere pressure

Geometry Level 2

A cube rests inside a sphere such that each vertex touches the sphere. The radius of the sphere is 6 cm . 6 \text{ cm}. Determine the volume of the cube.

If the volume of the cube can be expressed in the form of a 3 cm 3 a\sqrt{3} \text{ cm}^{3} , find the value of a a .


The answer is 192.

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2 solutions

Akshat Sharda
Dec 8, 2015

The longest diagonal of cube is equal to the diameter of the sphere.

Let the length of the side of cube be x x so the length of the longest diagonal is x 3 x\sqrt{3} .

x 3 = 6 × 2 x = 4 3 Volume = x 3 = ( 4 3 ) 2 = 192 3 \begin{aligned} \therefore x\sqrt{3} & =6×2 \\ x & =4\sqrt{3} \end{aligned} \\ \text{Volume}=x^3=(4\sqrt{3})^2=192\sqrt{3}

From the figure, the diameter of the sphere is the space diagonal of the cube which is equal to 2 ( 6 ) = 12 2(6)=12 . If a a is the side length of the cube, the space diagonal is a 3 a\sqrt{3} . So we have

12 = a 3 12=a\sqrt{3} \implies a = 12 3 a=\dfrac{12}{\sqrt{3}}

So the volume is v = a 3 = ( 12 3 ) 3 = 1728 3 3 2 = 1728 3 1 2 3 = 576 3 = 192 3 v=a^3=\left(\dfrac{12}{\sqrt{3}}\right)^3=\dfrac{1728}{3^{\frac{3}{2}}}=\dfrac{1728}{3^{\frac{1}{2}}\cdot 3}=\dfrac{576}{\sqrt{3}}=192\sqrt{3} .

The desired answer is 192 192 .

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