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Re: integer k ... DS [#permalink]
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I agree that the correct answer is A.

Is (k + 2)(k2 + 4k + 3) divisible by 4?

\((k + 2)(k^2 + 4k + 3)\)
\(= (k + 1)(k + 2)(k + 3)\)

Now to simplify matters, one of the terms NEEDS to be divisible by 4 in order for the whole expression to be divisible by 4. Why?

In order for an integer to be divisible by 4, its product must contain two factors of 2. This would appear to be satisfied by two even numbers. In the instance that we had two even numbers (only possible in this case if k+1 and k+3 are even, then we would already have a number divisible by 4 since those numbers are two consecutive even numbers.

Statement 1: k is divisible by 8:

If k is divisible by 8, then k = 8n, where n is a positive integer. This immediately makes k+1 and k+3 odd numbers, which are not divisible by 4. Now we test the (k+2) term:

\(k+2 = 8n + 2 = 4(2n) + 2\)

As a result, if k is divisible by 8, the expression when divided by 4 will have a remainder of 2. Therefore we can can conclude it IS NOT divisible by 4, and Statement 1 is sufficient.

Statement 2: (k + 1)/3 is an odd integer:

We can represent an odd integer as 2n + 1, where n is an integer.
Therefore,
\((k+1)/3 = 2n + 1\)
\(k = 6n + 2\)

Subbing this into the original equation:

\((k+1)(k+2)(k+3)\)
\(= (6n + 3)(6n + 4)(6n + 5)\)

Since 6n + 3 and 6n + 5 both represent odd numbers, if 6n + 4 is divisible by 4, the expression is divisible by 4. Since we cannot determine this (if n = 1, it is not divisible by 4, if n = 2, it is divisible by 4, etc.), Statement B is insufficient.

Therefore, the correct answer is A.
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Re: For positive integer k, is the expressio [#permalink]
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1) if k is divisible by 8 (and by 4 too), then k+2 is even but isn't divisible by 4 or 8.
(k^2+4k+3) - an odd integer. ---> (even but not divisible by 4 or 8)*odd ---> the expression isn't divisible by 4.

2)k+1/3 is odd --> k+1 is odd --> k is even --> k+2 is even. if k+2:
- is divisible by 4, the expression is also divisible by 4
- isn't divisible by 4, the expression isn't divisible by 4
insufficient
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Re: For positive integer k, is the expression (k + 2)(k2 + 4k + [#permalink]
IMO D is the answer.

For second statement, if K + 1/3 or (K+1)/3, K is even. And so, in (K + 2) (K^2 + 4K + 3) = (K+2)(K+3)(K+1) = even x odd x odd

Am I correct?
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For positive integer k, is the expression (k + 2)(k^2 + 4k + [#permalink]
Bunuel wrote:
pretzel wrote:
IMO D is the answer.

For second statement, if K + 1/3 or (K+1)/3, K is even. And so, in (K + 2) (K^2 + 4K + 3) = (K+2)(K+3)(K+1) = even x odd x odd

Am I correct?


No, the correct answer is A.

For positive integer k, is the expression (k + 2)(k^2 + 4k + 3) divisible by 4?

\((k + 2)(k^2 + 4k + 3)=(k+1)(k+2)(k+3)\), so the expression is the product of three consecutive integers.


(1) k is divisible by 8 --> \(k=8n=even\) --> \((k+1)(k+2)(k+3)=odd*even*odd\). Now, \(k+2=8n+2\), though even, is not a multiple of 4 (it's 2 greater than a multiple of 8), therefore the expression is not divisible by 4. Sufficient.

(2) (k + 1)/3 is an odd integer --> \(k+1=3*odd=odd\) --> \(k=even\) --> \((k+1)(k+2)(k+3)=odd*even*odd\). Now, \(k+2=even\) may or may not be divisible by 8, therefore the expression may or may not be divisible by 8. For example, consider \(k=2\) and \(k=6\). Not sufficient.

Answer: A.

Hope it's clear.


Hello Bunuel - For Statement 2, why are we checking for divisibility by 8? I get that a number divisible by 8 is divisible by 4 too. But is there a specific reason to check for divisibility by 8?

Also, how is 6 a valid value for K in statement 2? (K + 1)/3 is not an odd integer for K=6.

Statement 2 - For me, the legit values for K to get a yes and a no would be 2 and 32. For K=2, (k+1)(k+2)(k+3) is divisible by 4 and for K=32, it's not.

Please correct me.
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Re: For positive integer k, is the expression (k + 2)(k^2 + 4k + [#permalink]
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VikasBaloni wrote:
Bunuel wrote:
pretzel wrote:
IMO D is the answer.

For second statement, if K + 1/3 or (K+1)/3, K is even. And so, in (K + 2) (K^2 + 4K + 3) = (K+2)(K+3)(K+1) = even x odd x odd

Am I correct?


No, the correct answer is A.

For positive integer k, is the expression (k + 2)(k^2 + 4k + 3) divisible by 4?

\((k + 2)(k^2 + 4k + 3)=(k+1)(k+2)(k+3)\), so the expression is the product of three consecutive integers.


(1) k is divisible by 8 --> \(k=8n=even\) --> \((k+1)(k+2)(k+3)=odd*even*odd\). Now, \(k+2=8n+2\), though even, is not a multiple of 4 (it's 2 greater than a multiple of 8), therefore the expression is not divisible by 4. Sufficient.

(2) (k + 1)/3 is an odd integer --> \(k+1=3*odd=odd\) --> \(k=even\) --> \((k+1)(k+2)(k+3)=odd*even*odd\). Now, \(k+2=even\) may or may not be divisible by 8, therefore the expression may or may not be divisible by 8. For example, consider \(k=2\) and \(k=6\). Not sufficient.

Answer: A.

Hope it's clear.


Hello Bunuel - For Statement 2, why are we checking for divisibility by 8? I get that a number divisible by 8 is divisible by 4 too. But is there a specific reason to check for divisibility by 8?

Also, how is 6 a valid value for K in statement 2? (K + 1)/3 is not an odd integer for K=6.

Statement 2 - For me, the legit values for K to get a yes and a no would be 2 and 32. For K=2, (k+1)(k+2)(k+3) is divisible by 4 and for K=32, it's not.

Please correct me.


For some reason, I was checking for divisibility by 8 instead of 4. I've fixed that now. Thank you for pointing it out.
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