Bài 4:
\(a,\dfrac{5}{21}+\dfrac{2}{9}+\dfrac{3}{21}=\dfrac{5+3}{21}+\dfrac{2}{9}=\dfrac{8}{21}+\dfrac{2}{9}=\dfrac{8.3+9.7}{63}=\dfrac{24+35}{63}=\dfrac{59}{63}\\ b,\dfrac{5}{12}+\dfrac{-3}{20}-\dfrac{7}{12}=\dfrac{5-7}{12}-\dfrac{3}{20}=\dfrac{-2}{12}-\dfrac{3}{20}\\ =\dfrac{-1}{6}-\dfrac{3}{20}=\dfrac{\left(-1\right).10-3.3}{60}=\dfrac{-10-9}{60}=\dfrac{-19}{60}\\ c,\dfrac{5}{16}-\dfrac{-3}{26}+\dfrac{11}{16}=\left(\dfrac{5}{16}+\dfrac{11}{16}\right)-\dfrac{-3}{26}=\dfrac{16}{16}+\dfrac{3}{26}=1+\dfrac{3}{26}=\dfrac{29}{26}\\ d,\dfrac{7}{18}-\dfrac{3}{16}-\dfrac{13}{16}=\dfrac{7}{18}-\left(\dfrac{3}{16}+\dfrac{13}{16}\right)=\dfrac{7}{18}-\dfrac{16}{16}=\dfrac{7}{18}-1=-\dfrac{11}{18}\\ g,-\dfrac{5}{20}-\dfrac{3}{-20}+\dfrac{1}{4}=-\dfrac{1}{4}+\dfrac{3}{20}+\dfrac{1}{4}=\left(\dfrac{1}{4}-\dfrac{1}{4}\right)+\dfrac{3}{20}=0+\dfrac{3}{20}=\dfrac{3}{20}\)
Bài 6:
a: \(3-x=\dfrac{1}{2}\)
=>\(x=3-\dfrac{1}{2}=\dfrac{6}{2}-\dfrac{1}{2}=\dfrac{5}{2}\)
b: \(\dfrac{1}{7}-x=\dfrac{3}{14}\)
=>\(x=\dfrac{1}{7}-\dfrac{3}{14}=\dfrac{2}{14}-\dfrac{3}{14}=-\dfrac{1}{14}\)
c: \(2\dfrac{4}{7}-x=5\)
=>\(\dfrac{18}{7}-x=5\)
=>\(x=\dfrac{18}{7}-5=-\dfrac{17}{7}\)
d: \(-\dfrac{3}{8}-x=\dfrac{1}{18}\)
=>\(x=-\dfrac{3}{8}-\dfrac{1}{18}=\dfrac{-27-4}{72}=-\dfrac{31}{72}\)
g: \(\dfrac{1}{3}-x=\dfrac{1}{2}-\dfrac{3}{4}\)
=>\(\dfrac{1}{3}-x=-\dfrac{1}{4}\)
=>\(x=\dfrac{1}{3}-\left(-\dfrac{1}{4}\right)=\dfrac{1}{3}+\dfrac{1}{4}=\dfrac{7}{12}\)
e: \(\dfrac{3}{11}-x=\dfrac{1}{2}+\dfrac{5}{8}\)
=>\(\dfrac{3}{11}-x=\dfrac{4}{8}+\dfrac{5}{8}=\dfrac{9}{8}\)
=>\(x=\dfrac{3}{11}-\dfrac{9}{8}=\dfrac{24-99}{88}=-\dfrac{75}{88}\)
Bài 5:
a: \(x+\dfrac{1}{3}=\dfrac{5}{3}\)
=>\(x=\dfrac{5}{3}-\dfrac{1}{3}=\dfrac{4}{3}\)
b: \(x+\dfrac{-2}{5}=\dfrac{3}{10}\)
=>\(x=\dfrac{3}{10}-\dfrac{-2}{5}=\dfrac{3}{10}+\dfrac{2}{5}=\dfrac{7}{10}\)
c: \(x-\dfrac{5}{12}=\dfrac{3}{16}\)
=>\(x=\dfrac{3}{16}+\dfrac{5}{12}=\dfrac{9+20}{48}=\dfrac{29}{48}\)
d: \(x-\dfrac{7}{-12}=\dfrac{3}{26}\)
=>\(x+\dfrac{7}{12}=\dfrac{3}{26}\)
=>\(x=\dfrac{3}{26}-\dfrac{7}{12}=\dfrac{-73}{156}\)
g: \(x+\dfrac{-11}{12}=2\dfrac{1}{2}\)
=>\(x-\dfrac{11}{12}=\dfrac{5}{2}\)
=>\(x=\dfrac{5}{2}+\dfrac{11}{12}=\dfrac{30}{12}+\dfrac{11}{12}=\dfrac{41}{12}\)
e: \(x-3\dfrac{3}{5}=\dfrac{1}{5}\)
=>\(x-\dfrac{18}{5}=\dfrac{1}{5}\)
=>\(x=\dfrac{19}{5}\)
