Bài 1:
a) \(\dfrac{3}{5}+\dfrac{-2}{5}\) b) \(\dfrac{-2}{3}+\dfrac{-3}{4}\) c) \(\dfrac{2}{5}:\dfrac{6}{7}.\dfrac{-1}{3}\) d) \(\dfrac{3}{5}.\dfrac{5}{4}-\dfrac{3}{5}.\dfrac{1}{4}\)
= \(\dfrac{3+\left(-2\right)}{5}\) = \(\dfrac{-8}{12}+\dfrac{-9}{12}\) = \(\dfrac{2}{5}.\dfrac{7}{6}.\dfrac{-1}{3}\) =\(\dfrac{3}{5}.\left(\dfrac{5}{4}-\dfrac{1}{4}\right)\)
= \(\dfrac{1}{5}\) = \(\dfrac{\left(-8\right)+\left(-9\right)}{12}\) = \(\dfrac{7}{15}.\dfrac{-1}{3}\) = \(\dfrac{3}{5}.\dfrac{4}{4}\)
= \(\dfrac{-17}{12}\) = \(\dfrac{-7}{45}\) = \(\dfrac{3}{5}\) . 1
= \(\dfrac{3}{5}\)
Bài 2:
a) x + 3,4 = 20,7 b) 2x - 21,2 = -15,35 . 4
x = 20,7 - 3,4 2x - 21,2 = -61,4
x = 17,3 2x = -61,4 + 21,2
Vậy x=17,3 2x = -40,2
x = -40,2 : 2
x = -20,1
Vậy x = -20,1
