Répondre :
[tex] -(\frac{5}{21}- \frac{4}{7} ) + \frac{1}{3} =-( \frac{5*7-4*21}{21*7} )+ \frac{1}{3} =- \frac{-49}{147}+ \frac{1}{3} = \frac{49}{147}+ \frac{1}{3}= \frac{49}{3*49}+ \frac{1}{3}[/tex]
[tex] =\frac{1}{3}+ \frac{1}{3}= \frac{2}{3} [/tex]
[tex]\frac{1}{2}+ \frac{13}{2}* \frac{1}{26}= \frac{1}{2}+ \frac{13*1}{2*26}= \frac{1}{2}+ \frac{1}{2*2}= \frac{1}{2}+ \frac{1}{4}= \frac{4}{8}+ \frac{2}{8}= \frac{6}{8}= \frac{3}{4} [/tex]
[tex] =\frac{1}{3}+ \frac{1}{3}= \frac{2}{3} [/tex]
[tex]\frac{1}{2}+ \frac{13}{2}* \frac{1}{26}= \frac{1}{2}+ \frac{13*1}{2*26}= \frac{1}{2}+ \frac{1}{2*2}= \frac{1}{2}+ \frac{1}{4}= \frac{4}{8}+ \frac{2}{8}= \frac{6}{8}= \frac{3}{4} [/tex]