Here we have calculated n which gives the number of terms of our A.P i.e.
the number of even terms between 1 and n. So IMO, the total number of terms between 1 and n should be 79*2 + 1 = 159. Thus IMO, n comes out to be 159 and not 79 which is just the number of
even terms between 1 and n.
Could someone throw more light on this please.
Thanks and Regards.
Quote:
Originally Posted by Retake_GMAT
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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