Find the sum to n terms of the sequence 1,-1,1,-1,.........,(-1)n+1
here a=1 , r=-1
Sn = a(1-rn )/1-r
= 1(1- (-1)n) / 2
= 1 - (-1)n /2
now two cases arise .........1. n is odd
2. n is even
when n odd Sn = 1+1 / 2 =1
when n is even Sn = 1-1 /2 =0