[tex]Bonjour;\\\\\\A=\dfrac{24}{35}-\dfrac{\dfrac{3}{5}-\dfrac{9}{7}}{2-\dfrac{11}{4}}=\dfrac{24}{35}-\dfrac{\dfrac{3\times7}{5\times7}-\dfrac{9\times5}{7\times5}}{\dfrac{8}{4}-\dfrac{11}{4}}=\dfrac{24}{35}-\dfrac{\dfrac{21}{35}-\dfrac{45}{35}}{\dfrac{8-11}{4}}\\\\\\=\dfrac{24}{35}-\dfrac{\dfrac{21-45}{35}}{\dfrac{8-11}{4}}=\dfrac{24}{35}-\dfrac{\dfrac{-24}{35}}{\dfrac{-3}{4}}=\dfrac{24}{35}-\dfrac{-24}{35}\times\dfrac{4}{-3}=\dfrac{24}{35}-\dfrac{-24\times4}{35\times(-3)}\\[/tex]
[tex]=\dfrac{24}{35}-\dfrac{-96}{-105}=\dfrac{24\times3}{35\times3}-\dfrac{96}{105}=\dfrac{72}{105}-\dfrac{96}{105}=\dfrac{72-96}{105}=\dfrac{-24}{105}\\\\\\=-\dfrac{24}{105}=-\dfrac{3\times8}{3\times35}=-\dfrac{8}{35}\ .[/tex]
[tex]B=\dfrac{7\times10^{-12}\times0,04\times10^{15}}{2\times10^{-4}\times2,5\times(10^5)^{-3}}=\dfrac{7\times10^{-12}\times0,04\times10^{15}}{2\times10^{-4}\times2,5\times10^{-15}}\\\\\\=\dfrac{7\times0,04\times10^{-12}\times10^{15}}{2\times2,5\times10^{-4}\times10^{-15}}=\dfrac{0,28\times10^{-12+15}}{5\times10^{-4-15}}\\\\\\=\dfrac{0,28\times10^{3}}{5\times10^{-19}}=\dfrac{0,28}{5}\times10^{3+19}=0,056\times10^{22}\\\\\\=5,6\times10^{-2}\times10^{22}=5,6\times10^{-2+22}=5,6\times10^{20}\ .[/tex]