a)
[tex]( - \frac{1}{3}) - ( - \frac{1}{3} ) = 0[/tex]
b)
[tex]( - \frac{1}{2} ) - \frac{1}{6} = \frac{ - 3 \times 1 - 1}{6} = ( - \frac{4}{6} ) = ( - \frac{2}{3} )[/tex]
