\[\frac{1}{4} = \left( - \frac{1}{2} \right)^{2};\]
\[\frac{1}{64} = \frac{1}{32 \cdot 2} = \frac{1}{2^{5} \cdot 2} = \frac{1}{2^{5 + 1}} =\]
\[= \frac{1}{2^{6}} = \left( - \frac{1}{2} \right)^{6}.\]