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by shashank.ism » Tue Feb 09, 2010 1:15 pm
The sum of the first n positive even integers is 79*80, where n is an odd number, then n=?

79
80
81
78
77
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by fibbonnaci » Tue Feb 09, 2010 11:48 pm
First term a=2, common difference d=2 since even number

therefore sum to first n numbers of Arithmetic progression would be

n/2(2a+(n-1)d)

= n/2(2*2+(n-1)*2)=n(n+1) and this is equal to 79*80

therefore n=79 which is odd
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