Đáp án:
Bài 1 :
$a) -\dfrac{2}{5} - (-\dfrac{3}{11})$
$ = -\dfrac{2}{5} + \dfrac{3}{11}$
$ = \dfrac{22}{55} + \dfrac{15}{55}$
$ = \dfrac{37}{55}$
$b) (-5 - \dfrac{1}{2}) : \dfrac{1}{2}$
$ =(\dfrac{-10}{2} - \dfrac{1}{2}) : \dfrac{1}{2}$
$ = \dfrac{-11}{2} : \dfrac{1}{2}$
$ =-11$
Bài 2 :
$a, A = (\dfrac{2}{3} . \dfrac{1}{4} - \dfrac{1}{3} . \dfrac{1}{2} ) : (\dfrac{2}{7} . \dfrac{3}{9} - \dfrac{2}{7} .\dfrac{3}{5})$
$ A = (\dfrac{1}{6} -\dfrac{1}{6}) : ( \dfrac{2}{7} - (\dfrac{3}{9} - \dfrac{3}{5})$
$ A = 0 : ( \dfrac{2}{7} - \dfrac{4}{15})$
$ A = 0$
$b, B = (-\dfrac{4}{5} +\dfrac{5}{7} : \dfrac{2}{3} + (-\dfrac{1}{3} +\dfrac{2}{7}) : \dfrac{2}{3}$
$ B = (\dfrac{2}{7} + \dfrac{5}{7}) : \dfrac{2}{3} + (-\dfrac{1}{3} -\dfrac{4}{5}) : \dfrac{2}{3}$
$ B = 1 : \dfrac{2}{3} - \dfrac{17}{15} : \dfrac{2}{3}$
$ B = \dfrac{3}{2} - \dfrac{17}{10}$
$ B = -\dfrac{1}{5}$
Bài 3 :
$A = \dfrac{1}{-2} + \dfrac{1}{-6} + \dfrac{1}{-12} + \dfrac{1}{-20} + \dfrac{1}{-30} + \dfrac{1}{-42} +\dfrac{1}{-56}$
$A = -(\dfrac{1}{2} + \dfrac{1}{6} + \dfrac{1}{12} + \dfrac{1}{20} +\dfrac{1}{30} +\dfrac{1}{42} +\dfrac{1}{56})$
$A = - (1 -\dfrac{1}{2} + \dfrac{1}{2} - \dfrac{1}{3} + \dfrac{1}{3} - \dfrac{1}{4} +\dfrac{1}{4} -\dfrac{1}{5}+\dfrac{1}{5} -\dfrac{1}{6} +\dfrac{1}{6} -\dfrac{1}{7} +\dfrac{1}{7} -\dfrac{1}{8})$
$ A = -(1 - \dfrac{1}{8})$
$ A = - (\dfrac{8}{8} - \dfrac{1}{8})$
$ A = - (\dfrac{7}{8})$
$ = -\dfrac{7}{8}$
