Tính nhanh: `(-2/3) + 3/4 - (-1/6) + (-2/5)`

Đáp án:

$C1:$ 

$(-\dfrac{2}{3})+\dfrac{3}{4}-(-\dfrac{1}{6})+(-\dfrac{2}{5})$

$=[(-\dfrac{2}{3})-(-\dfrac{1}{6})]+[\dfrac{3}{4}+(-\dfrac{2}{5})]$

$= (\dfrac{-4}{6}+\dfrac{1}{6})+(\dfrac{15}{20}+\dfrac{-8}{20})$

$= \dfrac{-1}{2}+\dfrac{7}{20}$

$= \dfrac{-10}{20}+\dfrac{7}{20}$

$= \dfrac{-3}{20}$

$\text{_____________________}$

$C2:$

$(-\dfrac{2}{3})+\dfrac{3}{4}-(-\dfrac{1}{6})+(-\dfrac{2}{5})$

 $= \dfrac{-40}{60}+\dfrac{45}{60}-(-\dfrac{10}{60})+(-\dfrac{24}{60})$

$= \dfrac{(-40)+45-(-10)+(-24)}{60}$

$= \dfrac{-9}{60}$

$= \dfrac{-3}{20}$