D

\(a,\dfrac{6}{5}=\dfrac{18}{x}\\ \Rightarrow x=18:\dfrac{6}{5}\\ \Rightarrow x=15\\ b,\dfrac{3}{4}=\dfrac{-21}{x}\\ \Rightarrow x=-21:\dfrac{3}{4}\\ \Rightarrow x=-28\\ c,\dfrac{2}{-7}=\dfrac{18}{x}\\ \Rightarrow x=18:\dfrac{2}{-7}\\ \Rightarrow x=-63\\ d,\dfrac{-5}{2}=\dfrac{10}{-x}\\ \Rightarrow x=-10:\dfrac{-5}{2}\\ \Rightarrow x=4\)

\(a,\dfrac{6}{5}=\dfrac{18}{x}\Rightarrow6.x=5.18=90\\ \Rightarrow6.x=90\\ \Rightarrow x=15\\ b,\dfrac{3}{4}=\dfrac{-21}{x}\Rightarrow3.x=4.21=84\\ \Rightarrow x=28\)

\(a,\dfrac{6}{5}=\dfrac{18}{x}=>x=\dfrac{18.5}{6}=\dfrac{90}{6}=15\)

\(b,\dfrac{3}{4}=-\dfrac{21}{x}=>x=-\dfrac{21.4}{3}=-\dfrac{84}{3}=24\)

\(c,\dfrac{2}{-7}=\dfrac{18}{x}=>x=\dfrac{18.-7}{2}=-\dfrac{126}{2}=-63\)

\(d,-\dfrac{5}{2}=\dfrac{10}{-x}=>-x=\dfrac{10.2}{-5}=\dfrac{20}{-5}=-4=>x=4\)

\(\frac{7}{8}\div\left(\frac{14}{3}+\frac{14}{4}\right)+\frac{10}{14}\)

\(=\frac{7}{8}\div\left(\frac{56}{12}+\frac{52}{12}\right)+\frac{10}{14}\)

\(=\frac{7}{8}\div\frac{108}{12}+\frac{10}{14}\)

\(=\frac{7}{8}\div9+\frac{5}{7}\)

\(=\frac{7}{8}\times\frac{1}{9}+\frac{5}{7}\)

\(=\frac{7}{72}+\frac{5}{7}\)

\(=\frac{49}{504}+\frac{360}{504}\)

\(=\frac{409}{504}\)

\(\frac{30}{42}\div\left(\frac{5}{7}-\frac{5}{21}\right)+\frac{6}{8}\)

\(=\frac{6}{7}\div\left(\frac{15}{21}-\frac{5}{21}\right)+\frac{3}{4}\)

\(=\frac{6}{7}\div\frac{10}{21}+\frac{3}{4}\)

\(=\frac{6}{7}\times\frac{21}{10}+\frac{3}{4}\)

\(=\frac{126}{70}+\frac{3}{4}\)

\(=\frac{504}{280}+\frac{210}{280}\)

\(=\frac{714}{280}\)

\(\left(6-\frac{14}{15}\right).\frac{50}{38}-\frac{8}{18}\)

\(=\left(\frac{90}{15}-\frac{14}{15}\right).\frac{25}{19}-\frac{4}{9}\)

\(=\frac{76}{15}.\frac{25}{19}-\frac{4}{9}\)

\(=\frac{76.25}{15.19}-\frac{4}{9}\)

\(=\frac{19.2.2.5.5}{3.5.19}-\frac{4}{9}\)

\(=\frac{20}{3}-\frac{4}{9}\)

\(=\frac{60}{9}-\frac{4}{9}\)

\(=\frac{56}{9}\)

\(\frac{4}{6}+\frac{5}{21}.\left(\frac{4}{5}-\frac{2}{6}\right)\)

\(=\frac{2}{3}+\frac{5}{21}.\left(\frac{24}{30}-\frac{10}{30}\right)\)

\(=\frac{2}{3}+\frac{5}{21}.\frac{14}{30}\)

\(=\frac{2}{3}+\frac{70}{630}\)

\(=\frac{420}{630}+\frac{70}{630}\)

\(=\frac{490}{630}\)

\(=\frac{7}{9}\)

\(5/7; -3/21; -8/15\)

\(75/105; -15/105; -56/105\)

Chúc em học giỏi

`1/3+ -1/4+1/5+ -1/6+1/7+1/6+1/(-5)+1/4+ -1/3`

`=(1/3-1/3)+(1/4-1/4)+(1/5-1/5)+(1/6-1/6)+1/7`

`=0+0+0+0+1/7`

`=1/7`

gấp ạ

\(\frac{x}{-\frac{3}{5}}=\frac{-10}{21}\)

\(\Leftrightarrow21x=-10\cdot\frac{-3}{5}\)

\(\Leftrightarrow21x=6\Leftrightarrow x=\frac{2}{7}\)

\(\frac{x}{\frac{41}{3}}=-25\)

\(\Leftrightarrow x=-25\cdot\frac{41}{3}=\frac{-1025}{3}\)

3.Đề sao thế nhỉ???

(=) (4/7-2/3) X =  - 1/21

(=)  (12/21-14/21) X = -1/21

(=) -2/21 X= -1/21

(=) X = 1/2

em nên gõ công thức trực quan để đề bài rõ ràng nhé