The value of the sum 13∑n=1(in+in+1) , where i = √−1 equals
i
i-1
-i
0
13∑n=1(in+in+1) = ( i + i2 + i3 + ..... + i13) + ( i2 + i3 + .......+ i14 )
= i(1−i13)1−i+i2(1−i13)1−i
= i(1−i1−i)+i2(1−i)(1−i)=i+i2=i−1 .