a) 2/5+3/4=23/20

7/12-2/7+1/12=8//21

 12/17-5/17-4/17=3/17

b) 675,39+342,14=1017,53

563,87+403,13-328,35=638,65

a)            2/5+3/4

           =8/20+15/20

           =21/20

             7/12-2/7+1/12

            =7/12+1/12-2/7

             =1/4-2/7

              =7/28-8/28

              =-1/28

                 12/17-5/17-4/17

               =12/17-9/17

               =3/17

b)              675,39+342,14

               =       1017,53

                   563,87+403,13-328,35

                =       967-328,35

                 =  638,65

\(A.4-\left(12-17\right)=17-\left(x-4+15\right)\)

\(\Leftrightarrow4-\left(-5\right)=17-\left(x+11\right)\)

\(\Leftrightarrow9=17-x-11\)

\(\Leftrightarrow6-x=9\)

\(\Leftrightarrow x=6-9\)

\(\Leftrightarrow x=-3\)

\(B.14-\left(12-x\right)=4-\left(15-13\right)\)

\(\Leftrightarrow14-12+x=4-2\)

\(\Leftrightarrow2+x=2\)

\(\Leftrightarrow x=2-2\)

\(\Leftrightarrow x=0\)

a) 2/5 + 3/4 =8/20 + 15/20 = 23/20

7/12 - 2/7 + 1/12 = (7/12 + 1/12) - 2/7 = 8/12 - 2/7 = 2/3 - 2/7 = 14/21 - 6/21 =8/21

12/17 - 5/17 - 4/17 = 7/17 - 4/17 = 3/17

b) 675,39 + 342,14 = 1017,53 

 563,87 + 403,13 - 328,35 = 967 - 328,35 = 638,65

 Chúc bạn học tốt

A  =\(\frac{2}{5}\times\frac{4}{7}+\frac{3}{5}\times\frac{4}{7}\)  B = \(\frac{17}{13}\times\frac{12}{17}-\frac{4}{13}\times\frac{12}{17}\)   C = \(\frac{9}{14}:\frac{6}{7}+\frac{5}{14}:\frac{6}{7}\)  D =\(\frac{9}{12}:\frac{7}{9}\times\frac{12}{19}\)

=  \(\frac{4}{7}\times\left(\frac{2}{5}+\frac{3}{5}\right)\)              =\(\left(\frac{17}{13}-\frac{4}{13}\right)\times\frac{12}{17}\)                = \((\frac{9}{14}+\frac{5}{14}):\frac{6}{7}\)         = \(\frac{9}{12}\times\frac{9}{7}\times\frac{12}{19}\)

=\(\frac{4}{7}\times1\)                                     = \(1\times\frac{12}{17}\)                                       = \(1:\frac{6}{7}\)                               = \(\frac{9\times9\times12}{12\times7\times19}\)

=\(\frac{4}{7}\)                                              =  \(\frac{12}{17}\)                                                =\(\frac{7}{6}\)                                    = \(\frac{9\times9}{7\times19}=\frac{81}{133}\)

   -  

12/17+8/17=20/17+19/17=39/17

\(=-\left(\dfrac{4}{13}\cdot\dfrac{5}{17}+\dfrac{12}{13}\cdot\dfrac{4}{17}+\dfrac{4}{13}\right)\)

=-4/13

2/5 + 3/4 = 23/40

7/12 - 2/7 + 1/12 =  25/84 + 1/12 = 8/21

12/17 - 5/17 - 4/17 =  3/17

OK

\(\frac{2}{5}+\frac{3}{4}=\frac{8}{20}+\frac{15}{20}=\frac{23}{20}\)

\(\frac{7}{12}-\frac{2}{7}+\frac{1}{12}=\left(\frac{7}{12}+\frac{1}{12}\right)-\frac{2}{7}=\frac{2}{3}-\frac{2}{7}=\frac{14}{21}-\frac{6}{21}=\frac{8}{21}\)

\(\frac{12}{17}-\frac{5}{17}-\frac{4}{17}=\frac{12-5-4}{17}=\frac{3}{17}\)

a: =-5/11-6/11+1=-11/11+1=0

b: =-13/17-13/21-4/17=-1-13/21=-34/21

b: \(=-\dfrac{5}{12}\cdot\dfrac{9}{20}\cdot\dfrac{7}{17}=\dfrac{-21}{272}\)

d: \(=\dfrac{13}{17}\left(-\dfrac{4}{5}-\dfrac{3}{4}\right)=\dfrac{13}{17}\cdot\dfrac{-31}{20}=\dfrac{-403}{340}\)