Question

47. Tính nhanh.

a) ; b) .

Answer 1

\(\dfrac{-5}{21}+\dfrac{-2}{21}+\dfrac{8}{24}=\dfrac{-7}{21}+\dfrac{8}{24}=\dfrac{-1}{3}+\dfrac{1}{3}=\dfrac{0}{3}=0\)\(\dfrac{-3}{7}+\dfrac{5}{13}+\dfrac{-4}{7}=(\dfrac{-3}{7}+\dfrac{-4}{7})+\dfrac{5}{13}=\dfrac{-7}{7}+\dfrac{5}{13}=\left(-1\right)+\dfrac{5}{13}=\dfrac{-13}{13}+\dfrac{5}{13}=\dfrac{-8}{13}\)

Answer 2

a) \(\dfrac{-3}{7}+\dfrac{5}{13}+\dfrac{-4}{7}=\left(\dfrac{-3}{7}+\dfrac{-4}{7}\right)+\dfrac{5}{13}\)

= -1+\(\dfrac{5}{13}\)

=\(\dfrac{-13}{13}+\dfrac{3}{13}\)

=\(\dfrac{-8}{13}\)

Answer 3

b)

=\(\left(\dfrac{-5}{21}+\dfrac{-2}{21}\right)+\dfrac{1}{3}\)

=\(\dfrac{-1}{3}+\dfrac{1}{3}\)

=0

Answer 4

a) \(\frac{-3}{7}+\frac{5}{13}+\frac{-4}{7}\)

\(=\left(\frac{-3}{7}+\frac{-4}{7}\right)+\frac{5}{13}\)

\(=\left(-1\right)+\frac{5}{13}\)

\(=\frac{-8}{13}\)

b) \(\frac{-5}{21}+\frac{-2}{21}+\frac{8}{24}\)

\(=\frac{-1}{3}+\frac{1}{3}\)

\(=0\)