9)
\(a)x+\dfrac{1}{3}=\dfrac{3}{4}\\ x=\dfrac{3}{4}-\dfrac{1}{3}\\ x=\dfrac{9}{12}-\dfrac{4}{12}\\ x=\dfrac{5}{12}\\ b)x-\dfrac{2}{5}=\dfrac{5}{7}\\ x=\dfrac{5}{7}+\dfrac{2}{5}\\ x=\dfrac{25}{35}+\dfrac{14}{35}\\ x=\dfrac{39}{35}\\ c)-x-\dfrac{2}{3}=-\dfrac{6}{7}\\ x=-\dfrac{2}{3}+\dfrac{6}{7}\\ x=-\dfrac{14}{21}+\dfrac{18}{21}\\ x=\dfrac{4}{21}\\ d)\dfrac{4}{7}-x=\dfrac{1}{3}\\ x=\dfrac{4}{7}-\dfrac{1}{3}\\ x=\dfrac{12}{21}-\dfrac{7}{21}\\ x=\dfrac{5}{21}\)
6)
a) \(\dfrac{-1}{21}+\dfrac{-1}{28}\) b) \(\dfrac{-8}{18}-\dfrac{15}{27}\) c) \(\dfrac{-5}{12}+0,75\) d) \(3,5-\left(-\dfrac{2}{7}\right)\)
= \(\dfrac{-28}{84}+\dfrac{-21}{84}\) = \(\dfrac{-4}{9}-\dfrac{5}{9}\) = \(\dfrac{-5}{12}+\dfrac{3}{4}\) = \(\dfrac{7}{2}+\dfrac{2}{7}\)
= \(\dfrac{-49}{84}\) = \(\dfrac{-9}{9}\) = \(\dfrac{-5}{12}+\dfrac{9}{12}\) = \(\dfrac{49}{14}+\dfrac{4}{14}\)
= \(\dfrac{-7}{12}\) = -1 = \(\dfrac{4}{12}\) = \(\dfrac{53}{14}\)
= \(\dfrac{1}{3}\)
7)
\(\dfrac{15}{26}\) là tổng của 2 số hữu tỉ âm: Ví dụ:\(\dfrac{15}{26}\) = \(\dfrac{11}{26}+\dfrac{2}{13}\)
\(\dfrac{15}{26}\) là hiệu của 2 số hữu tỉ dương: Ví dụ: \(\dfrac{15}{26}=\dfrac{5}{13}-\dfrac{-5}{26}\)
8)
a) \(\dfrac{3}{7}+\left(-\dfrac{5}{2}\right)+\left(-\dfrac{3}{5}\right)\) b) \(\left(-\dfrac{4}{3}\right)+\left(-\dfrac{2}{5}\right)+\left(-\dfrac{3}{2}\right)\)
= \(\dfrac{3}{7}-\dfrac{5}{2}-\dfrac{3}{5}\) = \(\dfrac{-4}{3}-\dfrac{2}{5}-\dfrac{3}{2}\)
= \(\dfrac{30}{70}-\dfrac{175}{70}-\dfrac{42}{70}\) = \(\dfrac{-40}{30}-\dfrac{12}{30}-\dfrac{45}{30}\)
= \(\dfrac{-187}{70}\) = \(\dfrac{-97}{30}\)
c) \(\dfrac{4}{5}-\left(-\dfrac{2}{7}\right)-\dfrac{7}{10}\) d) \(\dfrac{2}{3}-\left[\left(-\dfrac{7}{4}\right)-\left(\dfrac{1}{2}+\dfrac{3}{8}\right)\right]\)
= \(\dfrac{4}{5}+\dfrac{2}{7}-\dfrac{7}{10}\) = \(\dfrac{2}{3}-\left[\left(-\dfrac{7}{4}\right)-\left(\dfrac{4}{8}+\dfrac{3}{8}\right)\right]\)
=\(\dfrac{56}{70}+\dfrac{20}{70}-\dfrac{49}{70}\) = \(\dfrac{2}{3}-\left[\left(-\dfrac{7}{4}\right)-\dfrac{7}{8}\right]\)
= \(\dfrac{27}{70}\) = \(\dfrac{2}{3}-\left[\dfrac{-7}{4}-\dfrac{7}{8}\right]\)
= \(\dfrac{2}{3}-\left[\dfrac{-14}{8}-\dfrac{7}{8}\right]\)
= \(\dfrac{2}{3}-\dfrac{-21}{8}\)
= \(\dfrac{16}{24}-\dfrac{-63}{24}\)
= \(\dfrac{79}{24}\)