What is the smallest positive integer n for which $$\sqrt{n\cdot7!}\ is\ an\ integer?$$ A. 14
B. 35
C. 70
D. 105
E. 210
The OA is the option B.
How can I find the value of n? Should I try number by number? Experts, may you give me some help, please?
What is the smallest positive integer
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- EconomistGMATTutor
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Hello Vjesus12.
Let's see your question.
You could try number by number or solve it as follows:
$$\sqrt{n\cdot7!}=\sqrt{n\cdot7\cdot6\cdot5\cdot4\cdot3\cdot2\cdot1}=2\sqrt{n\cdot7\cdot(3\cdot2)\cdot5\cdot3\cdot2}$$ $$2\sqrt{n\cdot7\cdot2\cdot3\cdot5\cdot3\cdot2}=2\sqrt{n\cdot7\cdot4\cdot9\cdot5}=2\cdot2\cdot3\sqrt{n\cdot7\cdot5}=12\sqrt{n\cdot7\cdot5}.$$ Now, this last number is an interger when the prime factorizazion has an odd number of 5's and a 7's.
On the given list, the smallest number that holds this condition is 35.
So, the correct answer is [spoiler]B=35[/spoiler].
I hope this explanation may help you.
I'm available if you'd like a follow-up.
Regards.
Let's see your question.
You could try number by number or solve it as follows:
$$\sqrt{n\cdot7!}=\sqrt{n\cdot7\cdot6\cdot5\cdot4\cdot3\cdot2\cdot1}=2\sqrt{n\cdot7\cdot(3\cdot2)\cdot5\cdot3\cdot2}$$ $$2\sqrt{n\cdot7\cdot2\cdot3\cdot5\cdot3\cdot2}=2\sqrt{n\cdot7\cdot4\cdot9\cdot5}=2\cdot2\cdot3\sqrt{n\cdot7\cdot5}=12\sqrt{n\cdot7\cdot5}.$$ Now, this last number is an interger when the prime factorizazion has an odd number of 5's and a 7's.
On the given list, the smallest number that holds this condition is 35.
So, the correct answer is [spoiler]B=35[/spoiler].
I hope this explanation may help you.
I'm available if you'd like a follow-up.
Regards.
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A perfect square has a prime decomposition in which each exponent is a multiple of 2.VJesus12 wrote:What is the smallest positive integer n for which $$\sqrt{n\cdot7!}\ is\ an\ integer?$$ A. 14
B. 35
C. 70
D. 105
E. 210
The OA is the option B.
How can I find the value of n? Should I try number by number? Experts, may you give me some help, please?
7! = 7 x 6 x 5 x 4 x 3 x 2 = 2^4 x 3^2 x 5 x 7, so the smallest value of n must be 5 x 7 = 35.
Answer: B
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