The sum of the even numbers between 1 and n is 79*80, where n is an odd number, then n=?
THE FASTER YOU REPLY THE MAX YOU ARE LIKEL TO WIN....
Wednesday, November 21, 2007
QUESTION #1
Posted by Gaurav & Kunal at 10:25 PM
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n=159
Got late on answering, but if I'll still put the explanation if it is helpful to someone. Also please follow up if you find a better way of solving it.
The number is 79*80
Now, the sum of even numbers can be generalized as 2 * Sum(n), i.e. where Sum(n) = 1+2+3+...+n
Hence that is n*(n+1)/2.
Now,
79*80 = 2 * Sum(79)
Hence, n in this case is 2*79.
Since we are to consider "n" as an odd number, n = 2*79 + 1 = 159.
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