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If n is a positive integer, is n^3 n divisible by 4 ?

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If n is a positive integer, is n^3 n divisible by 4 ? [#permalink] New post 23 Feb 2011, 21:25
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If n is a positive integer, is n^3 – n divisible by 4 ?

(1) n = 2k + 1, where k is an integer.
(2) n^2 + n is divisible by 6.
[Reveal] Spoiler: OA

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Last edited by Baten80 on 24 Feb 2011, 02:52, edited 1 time in total.
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Re: OG DS 170Arithmetic [#permalink] New post 24 Feb 2011, 02:29
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Baten80 wrote:
170. If n is a positive integer, is n3 – n divisible by 4 ?
(1) n = 2k + 1, where k is an integer.
(2) n2 + n is divisible by 6.


If n is a positive integer, is n^3 – n divisible by 4 ?

n^3-n=n(n^2-1)=(n-1)n(n+1), so we are asked whether the product of 3 consecutive integers is divisible bu 4.

(1) n = 2k + 1, where k is an integer --> n=odd --> as n is odd then both n-1 and n+1 are even hence (n-1)n(n+1) is divisible by 4. Sufficient.

(2) n^2 + n is divisible by 6 --> if n=2 then n^3-n=6 and the answer is NO but if n=3 then n^3-n=24 and the answer is YES. Not sufficient.

Answer: A.

P. S. Baten80 please format the questions properly.
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Re: If n is a positive integer, is n^3 n divisible by 4 ? (1) n [#permalink] New post 18 Apr 2012, 03:11
In the statement one when we express n=2k+1, why cant we take k=0, in that case n=1 and the product will be zero...Can we say then zero is divisible by 4?
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Re: If n is a positive integer, is n^3 n divisible by 4 ? (1) n [#permalink] New post 18 Apr 2012, 03:14
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pavanpuneet wrote:
In the statement one when we express n=2k+1, why cant we take k=0, in that case n=1 and the product will be zero...Can we say then zero is divisible by 4?


Zero is a divisible by every integer, except zero itself. Or which is the same: zero is a multiple of every integer, except zero itself. Integer \(a\) is a multiple of integer \(b\) means that \(a\) is "evenly divisible" by \(b\), i.e., divisible by \(b\) without a remainder. Now, since zero/integer=integer then zero is a multiple of every integer (except zero itself).

Check Number Theory chapter of Math Book for more: math-number-theory-88376.html

Hope it helps.
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Re: If n is a positive integer, is n^3 n divisible by 4 ? (1) n [#permalink] New post 23 Apr 2012, 11:32
Great info on number theory, did not know that... have to brush up on the basics.
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If n is a positive integer, is n^3 n divisible by 4 ? [#permalink] New post 10 Jun 2012, 13:18
If n is a positive integer, is n^3 - n divisible by 4?

(1) n = 2K + 1, where k is an integer
(2) n^2 + n is divisible by 6


Vote for A

(A) n = 2K + 1 therefore n = {1,3,5,7,9,11 ......}

when n=3 then 3(9-1) is divisible by 4 sufficient
all values of n = {1,3,5,7,9,11 ......} will be divisiable by 4

Information sufficent

(B) n^2 + n is divisible by 6

n(n+1) = 6q (q = any multiple of 6)
n & (n+1) are +ve consecutive integers and therefore co-prime numbers

therefore when
n=6 (n+1) = 7 divisible by 6 but not by 4
n=12 (n+1) = 13 divisible by 6 and 4

Information not sufficent
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Re: If n is a positive integer, is n^3 n divisible by 4 ? (1) n [#permalink] New post 15 Jun 2012, 14:37
Bunuel wrote:
pavanpuneet wrote:
In the statement one when we express n=2k+1, why cant we take k=0, in that case n=1 and the product will be zero...Can we say then zero is divisible by 4?


Zero is a divisible by every integer, except zero itself. Or which is the same: zero is a multiple of every integer, except zero itself. Integer \(a\) is a multiple of integer \(b\) means that \(a\) is "evenly divisible" by \(b\), i.e., divisible by \(b\) without a remainder. Now, since zero/integer=integer then zero is a multiple of every integer (except zero itself).

Check Number Theory chapter of Math Book for more: math-number-theory-88376.html

Hope it helps.


Sorry Bunuel, just to make this clear

if k = 0 then n = 1 (2 * 0 + 1)

this means that (n-1)(n)(n+1) is 0 * 1 * 2

which is 0 (zero) and therefore divisible by 4 because zero is divisible by 4 ... right?
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Re: If n is a positive integer, is n^3 n divisible by 4 ? (1) n [#permalink] New post 15 Jun 2012, 18:02
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solarzj wrote:
Bunuel wrote:
pavanpuneet wrote:
In the statement one when we express n=2k+1, why cant we take k=0, in that case n=1 and the product will be zero...Can we say then zero is divisible by 4?


Zero is a divisible by every integer, except zero itself. Or which is the same: zero is a multiple of every integer, except zero itself. Integer \(a\) is a multiple of integer \(b\) means that \(a\) is "evenly divisible" by \(b\), i.e., divisible by \(b\) without a remainder. Now, since zero/integer=integer then zero is a multiple of every integer (except zero itself).

Check Number Theory chapter of Math Book for more: math-number-theory-88376.html

Hope it helps.


Sorry Bunuel, just to make this clear

if k = 0 then n = 1 (2 * 0 + 1)

this means that (n-1)(n)(n+1) is 0 * 1 * 2

which is 0 (zero) and therefore divisible by 4 because zero is divisible by 4 ... right?


Exactly so: if n=1 then (n-1)(n)(n+1)=0*1*2=0 and 0 is divisible by every positive integer including 4.
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Re: If n is a positive integer, is n^3 n divisible by 4 ? [#permalink] New post 15 Aug 2013, 20:14
X= n^3 - n = n(n^2-1) = (n-1)*n*(n+1)

REM(X/4)=?

(1).

n is odd => For every odd number there exists two even numbers in the expression (n-1) and (n+1)
Hence divisible by 4 . SUFFICIENT

(2).

n*(n+1) is DIV by 6

n=2 => X=1*2*3 = 6
REM(X/4)=> REM(6/4) = 2

n=3 => X=2*3*4 = 24
REM(X/4) => REM(24/4) = 0

Hence two remainders on two cases => INSUFFICIENT

Hence (A) !
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Re: If n is a positive integer, is n^3 n divisible by 4 ? [#permalink] New post 08 Feb 2015, 00:23
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Re: If n is a positive integer, is n^3 n divisible by 4 ? [#permalink] New post 08 Feb 2015, 02:50
Baten80 wrote:
If n is a positive integer, is n^3 – n divisible by 4 ?

(1) n = 2k + 1, where k is an integer.
(2) n^2 + n is divisible by 6.


(n^3 - n) mod 4 = 0 ?
n(n^2 - 1) mod 4 = 0 ?
(n-1)(n)(n+1) mod 4 = 0 ?

1)n = 2k+1
=> 4k(2k+1)(k+1)
this is div by 4. hence, sufficient.

2) n^2 + n mod 6 = 0
n(n+1) mod 6 = 0
so either n or n+1 must be divisible by 2
if n is div by 2 then original expression is not div by 4
if n+1 is div by 2 then original expression is div by 4
hence, insufficient.

A.
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Re: If n is a positive integer, is n^3 n divisible by 4 ?   [#permalink] 08 Feb 2015, 02:50
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