Réponse un peu longue mais très détaillée.
[tex]A = \dfrac{\dfrac{3}{5} + \dfrac{5}{3} }{-2+\dfrac{1}{4} } \\A = \dfrac{\dfrac{3 \times 3}{5 \times 3} + \dfrac{5 \times 5}{3\times 5} }{-2+\dfrac{1}{4} } \\A = \dfrac{\dfrac{9}{15} + \dfrac{25}{15} }{-2+\dfrac{1}{4} } \\A = \dfrac{\dfrac{9+25}{15}}{-2+\dfrac{1}{4} } \\A = \dfrac{\dfrac{34}{15}}{-2+\dfrac{1}{4} } \\A = \dfrac{\dfrac{34}{15}}{\dfrac{-2}{1}+\dfrac{1}{4} } \\A = \dfrac{\dfrac{34}{15}}{\dfrac{-2 \times 4}{1 \times 4}+\dfrac{1}{4} } \\[/tex]
[tex]A = \dfrac{\dfrac{34}{15}}{\dfrac{-8}{4}+\dfrac{1}{4} } \\A = \dfrac{\dfrac{34}{15}}{\dfrac{-8 + 1}{4}} \\A = \dfrac{\dfrac{34}{15}}{\dfrac{-7}{4}} \\A = \dfrac{34}{15} \times \dfrac{4}{-7}\\A = \dfrac{34 \times 4}{15 \times (-7)} \\A = \dfrac{136}{-105} \\A = - \dfrac{136}{105} \\[/tex]