Bonjour,
[tex]\dfrac{5}{16}+\dfrac{3}{4}+\dfrac{7}{12}=\dfrac{5*3}{16*3}+\dfrac{3*12}{4*12}+\dfrac{7*4}{12*4}=\dfrac{15}{48}+\dfrac{36}{48}+\dfrac{28}{48}=\dfrac{15+36+28}{48}=\dfrac{79}{48}[/tex]
[tex]-\dfrac{9}{7}-(3+\dfrac{2}{21})=-\dfrac{9*3}{7*3}-\dfrac{3*21}{21}-\dfrac{2}{21}=-\dfrac{27}{21}-\dfrac{63}{21}-\dfrac{2}{21}=-\dfrac{27-63-2}{21}=-\dfrac{92}{21}[/tex]
[tex]\dfrac{5}{8}-\dfrac{5}{6}+\dfrac{1}{3} =\dfrac{5*3}{8*3}-\dfrac{5*4}{6*4}+\dfrac{1*8}{3*8}=\dfrac{15}{24}-\dfrac{20}{24}+\dfrac{8}{24}=\dfrac{15-20+8}{24}=\dfrac{3}{24} =\dfrac{1*3}{8*3}=\dfrac{1}{8}[/tex]
