Question

Tính :
a) \(\dfrac{-1}{21}+\dfrac{-1}{28}\)                       b) \(\dfrac{-8}{18}-\dfrac{15}{27}\)
c) \(\dfrac{-5}{12}+0,75\)                      d) \(3,5-\left(-\dfrac{2}{7}\right)\)

Answer 1

a)\(\dfrac{-1}{21}+\dfrac{-1}{28}=\dfrac{-4-3}{84}=\dfrac{-7}{84}=\dfrac{-1}{12}\)

b)\(\dfrac{-8}{18}-\dfrac{15}{27}=\dfrac{-24-30}{54}=\dfrac{-54}{54}=-1\)

c)\(\dfrac{-5}{12}+0,75=\dfrac{-5}{12}+\dfrac{3}{4}=\dfrac{-5+9}{12}=\dfrac{4}{12}=\dfrac{1}{3}\)

d)\(3,5-\left(-\dfrac{2}{7}\right)=\dfrac{7}{2}+\dfrac{2}{7}=\dfrac{49+4}{14}=\dfrac{53}{14}\)

Answer 2

a)-\(\dfrac{1}{12}\)

b)-1

c)\(\dfrac{1}{3}\)

d)\(\dfrac{53}{14}\)

Answer 3

a) \(\dfrac{-1}{21}+\dfrac{-1}{28}=\dfrac{-4}{84}+\dfrac{-3}{84}=\dfrac{\left(-4\right)+\left(-3\right)}{84}\)

\(=\dfrac{-7}{84}=\dfrac{-1}{12}\)

b)\(\dfrac{-8}{18}-\dfrac{15}{27}=\dfrac{-4}{9}-\dfrac{5}{9}=\dfrac{\left(-4\right)+\left(-5\right)}{9}=-1\)

c) \(\dfrac{-5}{12}+0,75=\dfrac{-5}{12}+\dfrac{3}{4}=\dfrac{-5+9}{12}=\dfrac{4}{12}=\dfrac{1}{3}\)

d) \(3,5-\left(-\dfrac{2}{7}\right)=\dfrac{7}{2}+\dfrac{2}{7}=\dfrac{49+4}{14}=\dfrac{53}{14}=3\dfrac{11}{14}\)