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For the positive integers a, b, and k, a^kb means that a^k
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12 Dec 2012, 03:59
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For the positive integers a, b, and k, a^kb means that a^k is a divisor of b, but a^(k + 1) is not a divisor of b. If k is a positive integer and 2^k72, then k is equal to (A) 2 (B) 3 (C) 4 (D) 8 (E) 18
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Math Expert
Joined: 02 Sep 2009
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Re: For the positive integers a, b, and k, a^kb means that a^k
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12 Dec 2012, 04:02
Walkabout wrote: For the positive integers a, b, and k, a^kb means that a^k is a divisor of b, but a^(k + 1) is not a divisor of b. If k is a positive integer and 2^k72, then k is equal to
(A) 2 (B) 3 (C) 4 (D) 8 (E) 18 \(72=2^3*3^2\), so we have that 2^3 is a divisor of 72 and 2^4 is not. Thus 2^372, hence k=3. Answer: B.
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Re: For the positive integers a, b, and k, a^kb means that a^k
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14 Dec 2012, 23:44
Initially looking to the problem one may try to plugin the numbers one by one. Here, 2^2=4 is a divisor of 72 and 2^3=8 is also a divisor of 72. But, we have to choose only one answer. 72=2x2x2x3x3=2^3 *3^2 and it is given that 72/2^k = integer. Here, we can equate 2^3=2^k and hence k=3. But, in fact 2^2 is also a divisor of 72 hence 2 could also be the answer. But, since it is additionally given that k+1 is not a divisor i.e. 2 in this case does not satisfy the condition because 2+1=3 and 2^3 is a divisior of 72. where as 3 satisfies the condition i.e. 3+1= 4 turning into 2^4 which is not a divisor of 72. This is how only one answer choice is left which is equal to 3 = answer choice B.




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Re: For the positive integers a, b, and k, a^kb means that a^k
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14 Dec 2012, 03:20
Ans: 72= 2^3x3^2, since 2^3 is a divisor of 72 k can be 3. Also 2^4=(2^(k+1)) is not a divisor of 72 , therefore the answer is (B).
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Re: For the positive integers a, b, and k, a^kb means that a^k
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19 Dec 2012, 07:15
2^k has to be a factor of 72.
Factors of 72: 3^2 x 2^3
Hence, k = 3.
Answer B.



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Re: For the positive integers a, b, and k, a^kb means that a^k
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25 Jul 2013, 08:30
Prime factorization is also helpful to solve this problem. For ME it´s faster.. Probably it helps someone.
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PS 110  prime factorization.jpg [ 20.75 KiB  Viewed 38327 times ]



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Re: For the positive integers a, b, and k, a^kb means that a^k
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11 Sep 2014, 11:29
Walkabout wrote: For the positive integers a, b, and k, a^kb means that a^k is a divisor of b, but a^(k + 1) is not a divisor of b. If k is a positive integer and 2^k72, then k is equal to
(A) 2 (B) 3 (C) 4 (D) 8 (E) 18 72 = 2*2*2*3*3 72/ 2^k = int ; 72/2^k+1 not integer if k= 2 then divisible ,k=3 then also .. if K=3 then divisible , k=4 then not.. Answer =B



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Re: For the positive integers a, b, and k, a^kb means that a^k
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02 Mar 2015, 02:35
If we say that 2^k is a divisor of 72 could we also do this to solve the problem?
Do the prime factorization of 72: 2^3 * 3^3, and then create the following equation:
2^k * 3^3 = 2^3 * 3^3, so k = 3?
Then we are assuming that k is 72, but 72 is still a divisor of 72. So, could we also do that?
As I see it, it is just a more visual way to say what everyone said above. Right...?



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Re: For the positive integers a, b, and k, a^kb means that a^k
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28 Aug 2015, 21:28
Bunuel wrote: Walkabout wrote: For the positive integers a, b, and k, a^kb means that a^k is a divisor of b, but a^(k + 1) is not a divisor of b. If k is a positive integer and 2^k72, then k is equal to
(A) 2 (B) 3 (C) 4 (D) 8 (E) 18 \(72=2^3*3^2\), so we have that 2^3 is a divisor of 72 and 2^4 is not. Thus 2^372, hence k=3. Answer: B. Why is the answer not A? if k= 2 it is still a divisor of 72
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Re: For the positive integers a, b, and k, a^kb means that a^k
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29 Aug 2015, 02:16
harishbiyani8888 wrote: Bunuel wrote: Walkabout wrote: For the positive integers a, b, and k, a^kb means that a^k is a divisor of b, but a^(k + 1) is not a divisor of b. If k is a positive integer and 2^k72, then k is equal to
(A) 2 (B) 3 (C) 4 (D) 8 (E) 18 \(72=2^3*3^2\), so we have that 2^3 is a divisor of 72 and 2^4 is not. Thus 2^372, hence k=3. Answer: B. Why is the answer not A? if k= 2 it is still a divisor of 72 We need such k that 2^k IS a divisor of 72 but 2^(k+1) is NOT. k cannot be 2 because 2^(2+1) IS a divisor of 72.
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Re: For the positive integers a, b, and k, a^kb means that a^k
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29 Aug 2015, 02:52
Thanks Bunuel for your prompt reply.
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Re: For the positive integers a, b, and k, a^kb means that a^k
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14 Mar 2016, 08:18
Great Question.. Here only K=3 satisfies the condition such that 8 divides 72 and 16 does not Hence B
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Re: For the positive integers a, b, and k, a^kb means that a^k
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18 May 2016, 12:30
Walkabout wrote: For the positive integers a, b, and k, a^kb means that a^k is a divisor of b, but a^(k + 1) is not a divisor of b. If k is a positive integer and 2^k72, then k is equal to
(A) 2 (B) 3 (C) 4 (D) 8 (E) 18 Solution: This is called a "defined function" problem. The parallel lines mean intrinsically nothing, except to establish a relationship between a^k and b.We are given that a^k  b means: 1) b/a^k = integer 2) b/a^(k+1) ≠ integer Next we are given specific numbers 2^k  72, and we must use the pattern to determine k, using a = 2 and b = 72; thus, we know: 72/2^k = integer AND 72/2^(k+1) ≠ integer In order for 72/2^k = integer AND 72/2^(k+1) ≠ integer to be true, k must equal 3. If have trouble seeing how this works, we can plug 3 back in to prove it. When k = 3, we know: 1) 72/2^3 = 72/8 = 9, which IS an integer. AND 2) 72/2^(3+1) = 72/2^4 = 72/16 = 4 1/2, which is NOT an integer. The answer is B.
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Re: For the positive integers a, b, and k, a^kb means that a^k
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06 Jun 2016, 08:42
Walkabout wrote: For the positive integers a, b, and k, a^kb means that a^k is a divisor of b, but a^(k + 1) is not a divisor of b. If k is a positive integer and 2^k72, then k is equal to
(A) 2 (B) 3 (C) 4 (D) 8 (E) 18 2^k 72 means 2^k is a divisor of 72, but 2^(k+1) is not a divisor of 72 72 = 2^3*3^2 The maximum powers of 2 in 72 = 3 Hence 2^3 is a divisor of 72 and 2^4 is not a divisor of 72 k = 3 Correct Option: B



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Re: For the positive integers a, b, and k, a^kb means that a^k
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25 Mar 2017, 06:28
the formula says that 72 is devisible by 2^k, but not by 2^(k+1) so, let's factor 72, 2^3 * 3^2, k is 3 Answer is B



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Re: For the positive integers a, b, and k, a^kb means that a^k
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01 Apr 2017, 07:05
The main point is what if the power of 2 in the integer 72? Factorize 72 72=2^3*3^2 Just concerned with 2. The answer (thus) is 3.
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Re: For the positive integers a, b, and k, a^kb means that a^k
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07 Mar 2019, 09:24
Great job with tags here!



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Re: For the positive integers a, b, and k, a^kb means that a^k
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28 Apr 2019, 05:57
Can someone please explain what  means? It looks like absolute value, but I though absolute value only applied when you actually put something in between the bars?



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Re: For the positive integers a, b, and k, a^kb means that a^k
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10 May 2019, 12:23
Walkabout wrote: For the positive integers a, b, and k, a^kb means that a^k is a divisor of b, but a^(k + 1) is not a divisor of b. If k is a positive integer and 2^k72, then k is equal to
(A) 2 (B) 3 (C) 4 (D) 8 (E) 18 From answer choice, If k=3 72 is divisible by 2^3=8 When, 2^k+1=2^4=16 then 72/16 is not divisible. According to condition Answer is B Posted from my mobile device



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Re: For the positive integers a, b, and k, a^kb means that a^k
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10 May 2019, 12:28
harishbiyani8888 wrote: Bunuel wrote: Walkabout wrote: For the positive integers a, b, and k, a^kb means that a^k is a divisor of b, but a^(k + 1) is not a divisor of b. If k is a positive integer and 2^k72, then k is equal to
(A) 2 (B) 3 (C) 4 (D) 8 (E) 18 \(72=2^3*3^2\), so we have that 2^3 is a divisor of 72 and 2^4 is not. Thus 2^372, hence k=3. Answer: B. Why is the answer not A? if k= 2 it is still a divisor of 72 Because we have another condition that 2^k+1 is not a divisor of 72. So, if we consider option A then, it'll be 2^2+1=8 by which 72 is possible to divide. So option A is ignored ☺ Posted from my mobile device




Re: For the positive integers a, b, and k, a^kb means that a^k
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