Đáp án:
Bài 2 :
$a) -2 + \dfrac{-5}{8} = \dfrac{-16}{8} + \dfrac{-5}{8} = \dfrac{-21}{8}$
$b) \dfrac{2}{21} -(-4) =\dfrac{2}{21} + \dfrac{84}{21} = \dfrac{86}{21}$
$c)-3\dfrac{1}{2} -2\dfrac{1}{4} = \dfrac{-7}{2} -\dfrac{9}{4} = \dfrac{-14}{4} -\dfrac{9}{4} = \dfrac{-23}{4}$
$d) -15 . 2\dfrac{1}{3} = -15 . \dfrac{7}{3} = \dfrac{-105}{3} = -35$
$e) \dfrac{9}{7} : (-3) = \dfrac{9}{7} . \dfrac{1}{-3} = \dfrac{9}{-21} = -\dfrac{3}{7}$
$f) 4\dfrac{1}{5} : (-2\dfrac{4}{5}) = \dfrac{21}{5} : (-\dfrac{14}{5}) = \dfrac{21}{5} . (-\dfrac{5}{14}) = -\dfrac{3}{2}$
Bài 3 :
$a) 1\dfrac{4}{23} + \dfrac{5}{21} -\dfrac{4}{23} + 0,5 + \dfrac{16}{21}$
$ = \dfrac{27}{23} + \dfrac{5}{21} -\dfrac{4}{23} + 0,5 + \dfrac{16}{21}$
$=(\dfrac{27}{23} -\dfrac{4}{23}) + (\dfrac{5}{21} +\dfrac{16}{21}) + 0,5$
$ =1 + 1 +0,5$
$ = 2,5$
$b) \dfrac{-5}{7} . \dfrac{2}{11} +\dfrac{-5}{7}.+\dfrac{9}{11} +\dfrac{-2}{7}$
$ = -\dfrac{5}{7}.(\dfrac{2}{11} +\dfrac{9}{11} ) -\dfrac{2}{7}$
$ = -\dfrac{5}{7} . 1 -\dfrac{2}{7}$
$ = -\dfrac{5}{7} -\dfrac{2}{7}$
$ = -1$
