Cube-Sphere

Geometry Level 1

When 88 cubes are rearranged to form the cube-sphere in the diagram above, their total surface area decreases by 384.

Find the total surface area of the cube-sphere.

Note : The cube-sphere is solid.


The answer is 144.

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6 solutions

Let x x be the side length of one cube, S A S_A be the total surface area of the 88 88 small cubes, and S B S_B be the surface area of the cube-sphere.

Then,

S A = 88 ( 6 x 2 ) = 528 x 2 S_A=88(6x^2)=528x^2

S B = 6 ( 4 x ) 2 + x ( 2 x ) ( 4 ) ( 6 ) = 96 x 2 + 48 x 2 = 144 x 2 S_B=6(4x)^2+x(2x)(4)(6)=96x^2+48x^2=144x^2

Given in the problem that,

S A = S B + 384 S_A=S_B+384 \implies 528 x 2 = 144 x 2 + 384 528x^2=144x^2+384 \implies 384 x 2 = 384 384x^2=384 \implies x 2 = 1 x^2=1

Therefore, S B = 144 x 2 = 144 ( 1 ) = S_B=144x^2=144(1)= 144 \color{#D61F06}\large \boxed{144}

Rishabh Jain
Feb 17, 2016

Let a denote the edge length of the cube. We can see that the TSA of cube sphere is 6 × ( 24 a 2 ) = 144 a 2 6\times(24a^2)=144a^2 while the TSA of 88 cubes is 88 × 6 a 2 = 528 a 2 88\times 6a^2=528a^2 .
Decrement in SA = 384 = ( 528 144 ) a 2 = 384 a 2 =384=(528-144)a^2=384a^2 a = 1 \Rightarrow a=1 Hence TSA of cube sphere= 144 a 2 = 144 144a^2=\Large\boxed{\color{#D61F06}{144}} .

Bert Seegmiller
Apr 6, 2018

The wording of the problem caused some confusion. "When 88 cubes are rearranged " -- rearranged from what?

I supposed that the "rearrangement" could be from 88 single cubes, because it probably wasn't from a 4x4x5 rectangular solid, which is neither a cube nor a sphere.

My work consisted of multiplying the number of cubes (88) times the number of sides of a cube (6), and then subtracting the reduction in surface area (384).

88 × 6 384 = 144 88 \times 6 - 384 = 144

A visual inspection of the illustration shows that each "side" of the cube-sphere has an area of 24, times 6, equals 144.

24 × 6 = 144 24 \times 6 = 144

Let x x be the side length of the smaller cubes. Then the total surface area of the 88 88 smaller cubes is 6 ( 88 ) ( x 2 ) = 528 x 2 6(88)(x^2)=528x^2 .

The total surface area of the cube-sphere solid is 6 ( 4 x ) 2 + 24 ( x ) ( 2 x ) = 144 x 2 6(4x)^2+24(x)(2x)=144x^2

Then we have the equation

144 x 2 = 528 x 2 384 144x^2=528x^2-384

x 2 = 1 x^2=1

So the total surface area of the cube-sphere is 144 x 2 = 144 ( 1 ) = 144 144x^2=144(1)=\boxed{144}

Let x x be the side length of one small cube. Then our equation is

88 ( 6 x 2 ) 384 = 6 ( 4 x ) 2 + 4 ( x ) ( 2 x ) ( 6 ) 88(6x^2)-384=6(4x)^2+4(x)(2x)(6)

528 x 2 384 = 96 x 2 + 48 x 2 528x^2-384=96x^2+48x^2

384 x 2 = 384 384x^2=384

x = 1 x=1

So the desired surface area is 96 + 48 = 144 96+48=\boxed{144} .

We let a a be the side length of each cube. The surface area of one cube is 6 6 a 2 a^2 . The total surface area of 88 88 c u b e s cubes is ( 88 ) ( 6 a 2 ) = 528 a 2 (88)(6a^2)=528a^2 .

Based from the figure, the total surfaced area of the cube-sphere is 6 ( 4 a ) 2 6 ( 2 a ) ( 2 a ) + [ 2 ( 2 a ) ( a ) + 2 ( 2 a ) ( a ) + 2 a ( 2 a ) ] ( 6 ) = 96 a 2 24 a 2 + 72 a 2 = 144 a 2 6(4a)^2-6(2a)(2a)+[2(2a)(a)+2(2a)(a)+2a(2a)](6) = 96a^2-24a^2+72a^2=144a^2 . From the problem, 528 a 2 144 a 2 = 384 528a^2-144a^2=384 , it follows that a 2 = 1 a^2=1 . Therefore, the total surfaced area of the cube-sphere is 144 a 2 = 144 ( 1 ) = 144 144a^2=144(1)=144

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