Notice the following pattern,
[tex]\begin{gathered} 8=2^3,4=2^2,2=2^1,0=2^0 \\ \frac{1}{2}=2^{-1},\frac{1}{4}=\frac{1}{2^2}=2^{-2},... \end{gathered}[/tex]Therefore, the given sequence can be rewritten as shown below
[tex]8,4,2,1,\frac{1}{2},\frac{1}{4},\frac{1}{8},\frac{1}{16},...=2^3,2^2,2^1,2^0,2^{-1},2^{-2},2^{-3},2^{-4},...[/tex]Thus, the formula is
[tex]a_n=2^{4-n},n\ge1[/tex]