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.
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Let a denote the edge length of the cube. We can see that the TSA of cube sphere is
6
×
(
2
4
a
2
)
=
1
4
4
a
2
while the TSA of 88 cubes is
8
8
×
6
a
2
=
5
2
8
a
2
.
Decrement in SA
=
3
8
4
=
(
5
2
8
−
1
4
4
)
a
2
=
3
8
4
a
2
⇒
a
=
1
Hence TSA of cube sphere=
1
4
4
a
2
=
1
4
4
.
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).
8 8 × 6 − 3 8 4 = 1 4 4
A visual inspection of the illustration shows that each "side" of the cube-sphere has an area of 24, times 6, equals 144.
2 4 × 6 = 1 4 4
Let x be the side length of the smaller cubes. Then the total surface area of the 8 8 smaller cubes is 6 ( 8 8 ) ( x 2 ) = 5 2 8 x 2 .
The total surface area of the cube-sphere solid is 6 ( 4 x ) 2 + 2 4 ( x ) ( 2 x ) = 1 4 4 x 2
Then we have the equation
1 4 4 x 2 = 5 2 8 x 2 − 3 8 4
x 2 = 1
So the total surface area of the cube-sphere is 1 4 4 x 2 = 1 4 4 ( 1 ) = 1 4 4
Let x be the side length of one small cube. Then our equation is
8 8 ( 6 x 2 ) − 3 8 4 = 6 ( 4 x ) 2 + 4 ( x ) ( 2 x ) ( 6 )
5 2 8 x 2 − 3 8 4 = 9 6 x 2 + 4 8 x 2
3 8 4 x 2 = 3 8 4
x = 1
So the desired surface area is 9 6 + 4 8 = 1 4 4 .
We let a be the side length of each cube. The surface area of one cube is 6 a 2 . The total surface area of 8 8 c u b e s is ( 8 8 ) ( 6 a 2 ) = 5 2 8 a 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 ) = 9 6 a 2 − 2 4 a 2 + 7 2 a 2 = 1 4 4 a 2 . From the problem, 5 2 8 a 2 − 1 4 4 a 2 = 3 8 4 , it follows that a 2 = 1 . Therefore, the total surfaced area of the cube-sphere is 1 4 4 a 2 = 1 4 4 ( 1 ) = 1 4 4
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Let x be the side length of one cube, S A be the total surface area of the 8 8 small cubes, and S B be the surface area of the cube-sphere.
Then,
S A = 8 8 ( 6 x 2 ) = 5 2 8 x 2
S B = 6 ( 4 x ) 2 + x ( 2 x ) ( 4 ) ( 6 ) = 9 6 x 2 + 4 8 x 2 = 1 4 4 x 2
Given in the problem that,
S A = S B + 3 8 4 ⟹ 5 2 8 x 2 = 1 4 4 x 2 + 3 8 4 ⟹ 3 8 4 x 2 = 3 8 4 ⟹ x 2 = 1
Therefore, S B = 1 4 4 x 2 = 1 4 4 ( 1 ) = 1 4 4