Bunuel wrote:
What is the smallest positive integer x, such that 1,152x is a perfect cube?
A. 4
B. 6
C. 8
D. 12
E. 18
------ASIDE--------------------------------------
Let's examine a perfect CUBE.
1000 is a perfect cube since 1000 = 10^3
Now check out the prime factorization of 1000
1000 = (2)(2)(2)(5)(5)(5)
= [(2)(5)][(2)(5)][(2)(5)]
Notice that we can take the prime factorization of 1000 and divide the prime factors into THREE identical groups
-----NOW ONTO THE QUESTION------------------------
1,152 = (2)(2)(2)(2)(2)(2)(2)(3)(3)
So, 1152
x = (2)(2)(2)(2)(2)(2)(2)(3)(3)(
x)
Let's
test the answer choices....
A.
x = 4 We get: 1152
x = (2)(2)(2)(2)(2)(2)(2)(3)(3)(
4)
= (2)(2)(2)(2)(2)(2)(2)(3)(3)(
2)(
2)
In order for the above to be a perfect CUBE, we must be able to divide the prime factors into THREE identical groups.
Since we cannot do that here, we can ELIMINATE A
B.
x = 6 We get: 1152
x = (2)(2)(2)(2)(2)(2)(2)(3)(3)(
6)
= (2)(2)(2)(2)(2)(2)(2)(3)(3)(
2)(
3)
Can we divide the above prime factors into THREE identical groups?
NO! ELIMINATE B
C.
x = 8 We get: 1152
x = (2)(2)(2)(2)(2)(2)(2)(3)(3)(
8)
= (2)(2)(2)(2)(2)(2)(2)(3)(3)(
2)(
2)(
2)
Can we divide the above prime factors into THREE identical groups?
NO! ELIMINATE C
D.
x = 12 We get: 1152
x = (2)(2)(2)(2)(2)(2)(2)(3)(3)(
12)
= (2)(2)(2)(2)(2)(2)(2)(3)(3)(
2)(
2)(
3)
Can we divide the above prime factors into THREE identical groups?
YES!
1152
x = [(2)(2)(2)(3)][(2)(2)(2)(3)][(2)(2)(2)(3)]
So, if x = 12, then 1152x IS a perfect cube.
Answer: D
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