Réponse :
67
[tex]A=\frac{2}{7} -\frac{3}{7} *\frac{14}{9} \\ = \frac{2}{7} -\frac{3*7*2}{7*3*3} \\= \frac{2}{7} -\frac{2}{3} \\=\frac{6}{21} -\frac{14}{21} \\=-\frac{8}{21}[/tex]
[tex]B = -\frac{7}{8} *(\frac{2}{3} -1)\\= - \frac{7}{8} *(\frac{2}{3}-\frac{3}{3})\\= -\frac{7}{8} * - \frac{1}{3}\\=\frac{7}{24}[/tex]
[tex]C= \frac{4}{5} *\frac{1}{4}+\frac{2}{5}\\= \frac{4*1}{5*4}+\frac{2}{5}\\=\frac{1}{5}+\frac{2}{5}\\=\frac{3}{5}[/tex]
68
[tex]A = - \frac{2}{5} : \frac{3}{15}+\frac{2}{15}\\= -\frac{6}{15} :\frac{3}{15} +\frac{2}{15}\\= \frac{6}{15} * \frac{15}{3} +\frac{2}{15}\\= \frac{6*15}{15*3} +\frac{2}{15}\\= - \frac{6}{3} +\frac{2}{15}\\=-2 +\frac{2}{15}\\= -\frac{30}{15} +\frac{2}{15}\\= -\frac{28}{15}[/tex]
[tex]B= \frac{9}{2} - \frac{5}{2} : \frac{15}{8} \\= \frac{9}{2} - \frac{5}{2} * \frac{8}{15} \\= \frac{9}{2} -\frac{5*2*4}{2*5*3} \\= \frac{9}{2} - \frac{4}{3} \\= \frac{27}{6} - \frac{8}{6} \\ = \frac{19}{6}[/tex]
