Bonjour,
[tex]\mathrm{A} = \frac{4}{3}-\frac{5}{7} = \frac{28}{21}-\frac{15}{21} = \frac{13}{21}[/tex]
[tex]\mathrm{B} = \frac{5}{8}+\frac{13}{12} = \frac{15}{24}+\frac{26}{24} = \frac{41}{24}[/tex]
[tex]\mathrm{C} = \frac{3}{8} - \frac{5}{4} = \frac{3}{8}-\frac{10}{8} = -\frac{7}{8}[/tex]
