Bài 7:
a: =5/3+7-5/3=7
b: =-7/31+24/17+7/31=24/17
c: =3/7-1/5-3/7=-1/5
Bài 6 :
a) \(\dfrac{1}{2}+\dfrac{-3}{5}=\dfrac{1}{2}-\dfrac{3}{5}=\dfrac{5}{10}-\dfrac{6}{10}=-\dfrac{1}{10}\)
b) \(\dfrac{5}{2}+\dfrac{-9}{8}=\dfrac{5}{2}-\dfrac{9}{8}=\dfrac{20}{8}-\dfrac{9}{8}=\dfrac{11}{8}\)
c) \(\dfrac{5}{2-15}+\dfrac{6}{8}=-\dfrac{5}{13}+\dfrac{3}{4}=\dfrac{19}{52}\)
d) \(\dfrac{15}{25}+\dfrac{-27}{12}=\dfrac{3}{5}+\dfrac{-9}{4}=\dfrac{3}{5}-\dfrac{9}{4}=\dfrac{12}{20}-\dfrac{45}{20}=-\dfrac{33}{20}\)
Bài 7 :
a) \(\dfrac{5}{3}+\left(7+\dfrac{-5}{3}\right)=\dfrac{5}{3}+\left(7-\dfrac{5}{3}\right)=\dfrac{5}{3}+7-\dfrac{5}{3}=\left(\dfrac{5}{3}-\dfrac{5}{3}\right)+7=7\)
b) \(\dfrac{-7}{31}+\left(\dfrac{24}{17}+\dfrac{7}{31}\right)=\dfrac{-7}{31}+\dfrac{24}{17}+\dfrac{7}{31}=\left(\dfrac{-7}{31}+\dfrac{7}{31}\right)+\dfrac{24}{17}=0+\dfrac{24}{17}=\dfrac{24}{17}\)
c) \(\dfrac{3}{7}+\left(\dfrac{-1}{5}+\dfrac{-3}{7}\right)=\dfrac{3}{7}+\dfrac{-1}{5}-\dfrac{3}{7}=\left(\dfrac{3}{7}-\dfrac{3}{7}\right)+\dfrac{-1}{5}=-\dfrac{1}{5}\)
bài 6
\(a,\dfrac{1}{2}+\dfrac{-3}{5}=\dfrac{5}{10}+\dfrac{-6}{10}=\dfrac{-1}{10}\)
\(b,\dfrac{5}{2}+\dfrac{-9}{8}=\dfrac{20}{8}+\dfrac{-9}{8}=\dfrac{11}{8}\)
\(c,\dfrac{5}{2-15}+\dfrac{6}{8}=\dfrac{5}{-13}+\dfrac{6}{8}=-\dfrac{40}{104}+\dfrac{78}{104}=\dfrac{19}{52}\)
\(d,\dfrac{15}{25}+\dfrac{-27}{12}=\dfrac{3}{15}+\dfrac{-9}{4}=\dfrac{12}{60}+\dfrac{-135}{60}=\dfrac{-123}{60}=\dfrac{-41}{20}\)
Câu 7
\(a,\dfrac{5}{3}+\left(7+\dfrac{-5}{3}\right)=\dfrac{5}{3}+7+\dfrac{-5}{3}=\left(\dfrac{5}{3}+\dfrac{-5}{3}\right)+7=7\)
\(b,\dfrac{-7}{31}+\left(\dfrac{24}{17}+\dfrac{7}{31}\right)=\dfrac{-7}{31}+\dfrac{24}{17}+\dfrac{7}{31}=\left(\dfrac{-7}{31}+\dfrac{7}{31}\right)+\dfrac{24}{17}=\dfrac{24}{17}\)
\(c,\dfrac{3}{7}+\left(\dfrac{-1}{5}+\dfrac{-3}{7}\right)=\dfrac{3}{7}+\dfrac{-1}{5}+\dfrac{-3}{7}=\left(\dfrac{3}{7}+\dfrac{-3}{7}\right)+\dfrac{-1}{5}=\dfrac{-1}{5}\)
