Is k^2 odd?
(1) k - 1 is divisible by 2.
(2) The sum of k consecutive integers is divisible by k.
The number properties
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Statement 1: As (k - 1) is divisible by 2, k must be odd.nidhis.1408 wrote:Is k^2 odd?
(1) k - 1 is divisible by 2.
(2) The sum of k consecutive integers is divisible by k.
Hence, k² is also odd.
Sufficient
Statement 2: Say, k consecutive integers are a, (a + 1), (a + 2), ..., and (a + k - 1)
Hence, the sum of these k consecutive integers, S = ak + k(k - 1)/2 = k[a + (k - 1)/2]
Now, if S is divisible by k, then [a + (k - 1)/2] must be integer.
So, (k - 1)/2 must be integer.
So, (k - 1) must be even.
Hence, k must be odd.
Therefore, k² is also odd.
Sufficient
The correct answer is D.
Last edited by Anurag@Gurome on Mon Jun 25, 2012 11:50 am, edited 3 times in total.
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Dear Anurag,
Can you please explain again how B is sufficient by itself?
Sum of 3(odd) consecutive numbers 1 2 3 is 6 (even) and sum of 2(even) consecutive 1 2 is 3 (odd).
Thank you
Pranith.
Can you please explain again how B is sufficient by itself?
Sum of 3(odd) consecutive numbers 1 2 3 is 6 (even) and sum of 2(even) consecutive 1 2 is 3 (odd).
Thank you
Pranith.
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Thanks for pointing it out.
I've edited my post.
I've edited my post.
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Dear Anurag,
I think IMO is A.
Sum of K consecutive integers (K*(K+1)/2) is always divisible by K regardless of K being even or odd.
So we cannot say if K^2 is even or odd.
And, sum of even number of consecutive integers is not always odd (1 + 2 + 3 + 4 = 10)
What am I missing?
I think IMO is A.
Sum of K consecutive integers (K*(K+1)/2) is always divisible by K regardless of K being even or odd.
So we cannot say if K^2 is even or odd.
And, sum of even number of consecutive integers is not always odd (1 + 2 + 3 + 4 = 10)
What am I missing?
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K(K + 1)/2 is the sum of first K integers, which is not same as sum of K consecutive integers.TimeforGMAT wrote:Sum of K consecutive integers (K*(K+1)/2) is always divisible by K regardless of K being even or odd.
So we cannot say if K^2 is even or odd.
For the rest part see my edited post above. I've tried to explain it with algebra.
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