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Number System

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Number System

by CaptainHaddock » Mon Oct 31, 2011 10:28 am
The sum of the first N natural numbers is given by N(N+1)/2. What is the sum of the first N/2 even natural numbers if N is even?

A. N(N-2)/4
B. N(N-1)/4
C. N^2/4
D. N(N+1)/4
E. N(N+2)/4

Would like to know the methodology. OA is E
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by gmatblood » Mon Oct 31, 2011 10:34 am
CaptainHaddock wrote:The sum of the first N natural numbers is given by N(N+1)/2. What is the sum of the first N/2 even natural numbers if N is even?

A. N(N-2)/4
B. N(N-1)/4
C. N^2/4
D. N(N+1)/4
E. N(N+2)/4

Would like to know the methodology. OA is E
I knew it with the formula, E is correct
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by shankar.ashwin » Mon Oct 31, 2011 11:16 am
If you can't deduce a pattern, substitute numbers.
Let N =2. First 2 even numbers are (2,4)
N/2 - 1 . The first term = 2.

Now substitute N=2 in answer choices and see which one gives you 2. only E does.
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