$a_{1}q^{3} = 1$ $a_{1}q^{6} = 8$ $\Rightarrow \frac{a_{1}q^{6}}{a_{1}q^{3}} = 8 \Rightarrow q^{3} = 8 \Rightarrow q = 2$ $a_{1}q^{3} = 1 \Rightarrow a_{1}(8) = 1 \Rightarrow a_{1} = 1/8$ $a_{n} = a_{1}q^{n-1} = (1/8)(2^{n-1}) = 1/2^{3} \times 2^{n-1} \Rightarrow a_{n} = 2^{n-4}$