ANSWER
first is the geometric series
EXPLANATION
a geometric series is a sequence of numbers whose first term is non zero and each of the succeeding terms is equal to the Prcedding term multiply by a constant number.
if a = first term
and constant number = d
then
GP is given by
a,a r , a r^2 ,a r^3 .............and so on
in first
a = 1/2
and d = 1/2
and hence it is an GP
