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

samal · 07-23-2008, 04:17 PM

I just verified, the answer is 159 and not 79.
Consider this
sum1 = 2 + 4 + 6 + 8 ....+ 76 + 78 (even numbers less than 79)
= 2(1 + 2 + 3 + ...+38 + 39)
= 2*39*(40/2)
= 39.40

Now consider this
sum2 = 2 + 4 + 6..+ 156 + 158 (even numbers less than 159)
= 2(1 + 2 + 3 + 4.....+ 78 + 79)
= 2 * 79 * (80/2)
= 79 * 80

So n is 159 and not 79.

The mistake done in the sumation of series was in substituing the value of n (how many terms are there in the series), it is not n and not n-1 but rather n/2 and then the answer would come as 159.

07-23-2008, 04:17 PM	#7 (permalink)
samal Within my grasp! Join Date: Mar 2008 Posts: 222	I just verified, the answer is 159 and not 79. Consider this sum1 = 2 + 4 + 6 + 8 ....+ 76 + 78 (even numbers less than 79) = 2(1 + 2 + 3 + ...+38 + 39) = 239(40/2) = 39.40 Now consider this sum2 = 2 + 4 + 6..+ 156 + 158 (even numbers less than 159) = 2(1 + 2 + 3 + 4.....+ 78 + 79) = 2 * 79 * (80/2) = 79 * 80 So n is 159 and not 79. The mistake done in the sumation of series was in substituing the value of n (how many terms are there in the series), it is not n and not n-1 but rather n/2 and then the answer would come as 159.