1, 1/2, 1/4, 1/8, 1/16,.....

in the sequence the first term is one and each term after the first is half the proceeding term. which of following answers represents the nth term of this sequence, where n is positive?

its 1/2^n-1because 1/2^3= 1/8 1/2^2=1/2 ------- but my coworker said its 1/2^n

Can someone explain to me if i am doing this correctly.