TestMagic Forums - View Single Post - Sum of even numbers:-

samal · 07-23-2008, 03:02 AM

Got this question from a compilation.
Q. The sum of the even numbers between 1 and n is 79*80, where n is an odd number, then n=?

My approach has been:-
The set of numbers give is (1,2,3,4,5,6,....n-1,n)

So the num of even numbers in that set is
sum = 2 + 4 + 6 + 8 ...(n-1)
=2*(1+2+3+4...+(n-1)/2)
=2*(((n-1)/2)*((n-1)/2)+1))/2 (sum of ni consecutive numbers)
= ((n-1)*(n+1))/4 = 79*80

Solving gives n = 159.

But the answer given is 79. Can somebody confirm my answer?

07-23-2008, 03:02 AM	#1 (permalink)
samal Within my grasp! Join Date: Mar 2008 Posts: 222	Sum of even numbers:- Got this question from a compilation. Q. The sum of the even numbers between 1 and n is 7980, where n is an odd number, then n=? My approach has been:- The set of numbers give is (1,2,3,4,5,6,....n-1,n) So the num of even numbers in that set is sum = 2 + 4 + 6 + 8 ...(n-1) =2(1+2+3+4...+(n-1)/2) =2(((n-1)/2)((n-1)/2)+1))/2 (sum of ni consecutive numbers) = ((n-1)(n+1))/4 = 7980 Solving gives n = 159. But the answer given is 79. Can somebody confirm my answer?