Đáp án:
Giải thích các bước giải:
$\dfrac{5}{31}.\dfrac{21}{25}+\dfrac{5}{31}.\dfrac{-7}{10}-\dfrac{5}{31}.\dfrac{9}{20}$
$=\dfrac{5}{31}.\left (\dfrac{21}{25}-\dfrac{7}{10}-\dfrac{9}{20} \right )$
$=\dfrac{5}{31}.\left (\dfrac{84}{100}-\dfrac{70}{100}-\dfrac{45}{100} \right )$
$=\dfrac{5}{31}.\dfrac{-31}{100}$
$=\dfrac{-1}{20}$