If n is a positive integer and n squared is divisible by 72, then the largest possible integer that must divide n is
a. 6
b. 12
c. 24
d. 36
e. 48
Number Properties HELP!!
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- Anurag@Gurome
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Picking Number Approachshriti wrote:If n is a positive integer and n squared is divisible by 72, then the largest possible integer that must divide n is
a. 6
b. 12
c. 24
d. 36
e. 48
- Least possible value of n² such that n² is divisible by 72 is 72*2 = 144
Hence, minimum possible value of n = 12.
Largest possible integer that divides n is 12.
- n² is divisible by 72
Hence we can write n² as 72k, where k is an positive integer.
Now, n = √n² = √(72k) = √[(2)*(36)*k] = 6√(2k)
Now for n to be an integer, k must be an even multiple of a perfect square.
Hence, we can write k = 2m², where is a positive integer.
Now, n = 6√(2k) = 6√(2*2*m²) = 12m
Hence, largest possible integer that divides n is 12
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The prime factor of 72 are -> 2, 2, 2, 3, 3shriti wrote:If n is a positive integer and n squared is divisible by 72, then the largest possible integer that must divide n is
a. 6
b. 12
c. 24
d. 36
e. 48
n² is divisible by 72 HENCE 2, 2, 2, 3, 3, ...? (2², 2, 3²)are prime factors of n² ("...?" because there could be more prime factors)
Because n² must have at least (2², 2, 3²) in its prime factors, and n² = n * n, we can distribute this factors to determine the prime factors of n:
n²
=
n -> 2 * 3 * 2
*
n -> 2 * 3 * 2 We must fill this space with "2" to obtain two equal values of n
[spoiler]So the largest number that must divide n is 2 * 3 * 2, which is 12 (1, 2, 3, and 6 could divide n but 12 is the largest one)
Correct Answer is ...B...[/spoiler]
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Please, why is 48 not the answer? 48^2 is divisible by 72, and 48 divides 'n'.
By the way, what is the source of this question?
By the way, what is the source of this question?
I'm really old, but I'll never be too old to become more educated.
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n>0 (n is integer)shriti wrote:If n is a positive integer and n squared is divisible by 72, then the largest possible integer that must divide n is
a. 6
b. 12
c. 24
d. 36
e. 48
N^2/72=i (i is integer)
prime factorization of 72
72--2--36
36--2--18
18--2--9
9---3--3
3---3--1
72=2^3 * 3^2; to make 72 the perfect square we multiply by 2^1 --> 2^4 * 3^2=(4*3)^2
12^2 is divisible by 72; n=12
@Tomada: 72=2^3 * 3^2; to make 72 the perfect square we multiply by 2^1 and 3^2 --> 2^4 * 3^4=(4*9)^2
36^2 is divisible by 72; n=36
@Tomada: 72=2^3 * 3^2; to make 72 the perfect square we multiply by 2^3 --> 2^6 * 3^2=(8*3)^2
24^2 is divisible by 72; n=24
...
so basically we are missing one important statement in this problem:
If n is a positive integer and n squared is divisible by 72, then the largest possible integer that must divide the lowest value of n is
a. 6
b. 12
c. 24
d. 36
e. 48
the correct answer is B then
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@Tomada -->
Anurag@Gurome wrote:Picking Number Approachshriti wrote:If n is a positive integer and n squared is divisible by 72, then the largest possible integer that must divide n is
a. 6
b. 12
c. 24
d. 36
e. 48Algebraic Approach:
- Least possible value of n² such that n² is divisible by 72 is 72*2 = 144
Hence, minimum possible value of n = 12.
Largest possible integer that divides n is 12.The correct answer is B.
- n² is divisible by 72
Hence we can write n² as 72k, where k is an positive integer.
Now, n = √n² = √(72k) = √[(2)*(36)*k] = 6√(2k)
Now for n to be an integer, k must be an even multiple of a perfect square.
Hence, we can write k = 2m², where is a positive integer.
Now, n = 6√(2k) = 6√(2*2*m²) = 12m
Hence, largest possible integer that divides n is 12
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The trick here is "MUST", because the smallest possible value of n² is 144 (2² * 2² * 3²) the largest possible divisor of n MUST be 12. In this case 12 is not divisible by 48.tomada wrote:Please, why is 48 not the answer? 48^2 is divisible by 72, and 48 divides 'n'.
By the way, what is the source of this question?
It could be that n² is greater than 144 but we are not sure about that!!! in that case n could be divisible by 48
I think this problem is from Manhattan GMAT number properties.
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Thanks Maaj - just to build on what you said earlier, I think we have to take the whole phrase in itself..."the largest possible integer that MUST divide n is"...because of "must" it's like saying "the lowest possible integer that could divide n is"...is that correct? Is this a common pattern found in gmats?