\[\left( - 1\frac{1}{2} \right)^{3} - \left( \frac{1}{2} \right)^{3} = \left( - \frac{3}{2} \right)^{3} - \frac{1}{8} =\]
\[= - \frac{27}{8} - \frac{1}{8} = - \frac{28}{8} =\]
\[= - 3\frac{1}{2} = - 3,5\]