A geometric sequence has a initial value of 1/2 and a common ratio of 8. Write an exponential function to represent this sequence.

a. f(x) = 8 * (1/2)^x
b. f(x) = 8 * (1/2)^x-1
c. f(x) = 1/2 * 8^x
d. f(x) = 1/2 * 8^x-1