[tex] {a}^{2} - 4b {}^{3} = ( - 4) {}^{2} - 4 \times ( \frac{1}{2} ) {}^{3} = 16 - 4 \times \frac{1}{8} = 16 - \frac{1}{2} = 15 \frac{2}{2} - \frac{1}{2} = 15 \frac{1}{2} = 15.5[/tex]