không biết

a) (689+875)+125=1553+125=1678

581+(878+419)=581+1297=1878

b) (\(\dfrac{2}{7}\)+\(\dfrac{4}{9}\))+\(\dfrac{5}{7}\)=(\(\dfrac{18}{63}\)+\(\dfrac{28}{63}\))+\(\dfrac{45}{63}\)=\(\dfrac{46}{63}\)+\(\dfrac{45}{63}\)=\(\dfrac{13}{9}\)

\(\dfrac{17}{11}\)+(\(\dfrac{7}{15}\)+\(\dfrac{5}{11}\))=\(\dfrac{17}{11}\)+\(\dfrac{5}{11}\)+\(\dfrac{7}{15}\)=2+\(\dfrac{7}{15}\)=\(\dfrac{37}{15}\)

c)5,87+28,69+4,13=38,69

83,75+46,98+6,25=136,98

a) 889972 + 96308=986280

b) \(\dfrac{5}{6}\)+\(\dfrac{7}{12}\)=\(\dfrac{10}{12}\)+\(\dfrac{7}{12}\)=\(\dfrac{17}{12}\)

c) 3+\(\dfrac{5}{7}\)=\(\dfrac{21}{7}\)+\(\dfrac{5}{7}\)=\(\dfrac{26}{7}\)

d) 926,83 +549,67=1476,5