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A perfect square is defined as the square of an integer and [#permalink]
22 Jan 2011, 03:49

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A perfect square is defined as the square of an integer and a perfect cube is defined as the cube of an integer. How many positive integers n are there such that n is less than 1,000 and at the same time n is a perfect square and a perfect cube?

Re: A perfect square is defined as the square of an integer and [#permalink]
22 Jan 2011, 08:12

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amod243 wrote:

A perfect square is defined as the square of an integer and a perfect cube is defined as the cube of an integer. How many positive integers n are there such that n is less than 1,000 and at the same time n is a perfect square and a perfect cube?

(A) 2 (B) 3 (C) 4 (D) 5 (E) 6

don't know the OA

Given: positive integer \(n\) is a perfect square and a perfect cube --> \(n\) is of a form of \(n=x^6\) for some positive integer \(x\) --> \(0<x^6<10^3\) --> \(0<x^2<10\) --> \(x\) can be 1, 2 or 3 hence \(n\) can be 1^6, 2^6 or 3^6.

Answer: B.

amod243 wrote:

hi Mackieman

Agreed with your explanation. but i think we need to consider 0 also???

because 0 also satisfy the condition..

\(n\) can not be 0 as given that \(n\) is a positive integer. _________________

Re: A perfect square is defined as the square of an integer and [#permalink]
18 Jul 2014, 23:30

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If the square root of any of these numbers results in an integer then they will be both perfect square and perfect cube.

By looking at the numbers we can see that only three numbers results in integer, sqrt(1) =1, sqrt(4) = 2 and sqrt(9) = 3 => the answer is 3.

(ex sqrt(8^3) = sqrt(8*8*8) = sqrt(8)*sqrt(8)*sqrt(8) = 8*sqrt(8) = not integer)

Didn't get your explanation. 5^3 = 125 , 8^3 = 512 , 6^3 = 216.

IMO , the answer should be 2 as there are only two numbers which can come as perfect square as well as cube between 1 and 1000. The numbers would be 1 and 64

If the square root of any of these numbers results in an integer then they will be both perfect square and perfect cube.

By looking at the numbers we can see that only three numbers results in integer, sqrt(1) =1, sqrt(4) = 2 and sqrt(9) = 3 => the answer is 3.

(ex sqrt(8^3) = sqrt(8*8*8) = sqrt(8)*sqrt(8)*sqrt(8) = 8*sqrt(8) = not integer)

Didn't get your explanation. 5^3 = 125 , 8^3 = 512 , 6^3 = 216.

IMO , the answer should be 2 as there are only two numbers which can come as perfect square as well as cube between 1 and 1000. The numbers would be 1 and 64

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