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Joe has exactly enough paint to paint the surface of a cube whose main diagonal is 2. It turns out that this is also exactly enough paint to paint the surface of a sphere. Find the radius of the sphere.

 Apr 16, 2022
 #1
avatar+9369 
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The main diagonal of a cube is \(\sqrt 3\) times longer than its side length.

Therefore, the side length of cube = \(\dfrac2{\sqrt 3}\).

Now, we can calculate the surface area by the formula \(\text{surface area of cube} = 6(\text{side length})^2\).

\(\text{surface area of cube} = 6\left(\dfrac2{\sqrt 3}\right)^2 = 8\)

Since the amount of paint used is exactly the same as the amount of paint to paint the surface of a sphere, the surface area of the sphere is the same as that of the cube.

\(\text{surface area of sphere} = 8\).

 

Let r be the radius of the sphere. Then,

 

\(4\pi r^2 = 8\\ r^2 = \dfrac2\pi\\ r = \sqrt{\dfrac2\pi}\)

 

Therefore, radius of sphere = \(\sqrt{\dfrac{2}\pi}\).

 Apr 16, 2022
 #2
avatar+1384 
+1

The main diagonal is 2. Because it is a cube, all the side lengths are the same, and we have the equation: \(\sqrt{x^2+x^2+x^2}=2\)

 

Solving, we find \(x^2 = {4 \over3}\). The surface area is \(6x^2\), meaning the surace area of the cube is \(8\).

 

Now, we have the equation: \(4\pi r=8\)

 

Diving both sides by 4, we get: \(\pi r = 2\)

 

Solving for r, we find \(r = \color{brown}\boxed{\sqrt{2\over \pi}}\) 

 Apr 16, 2022

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