Bài làm:

1) \(\frac{3}{5}\div\frac{2x}{15}=\frac{1}{2}\div\frac{4}{5}\)

\(\Leftrightarrow\frac{9}{2x}=\frac{5}{8}\)

\(\Rightarrow10x=72\)

\(\Leftrightarrow x=\frac{36}{5}\)

2) \(-\frac{4}{2,5}\div\frac{3}{5}=\frac{1}{5}\div x\)

\(\Leftrightarrow\frac{1}{5}\div x=-\frac{8}{3}\)

\(\Rightarrow x=-\frac{3}{40}\)

3) \(0,12\div3=2x\div\frac{3}{5}\)

\(\Leftrightarrow\frac{1}{25}=\frac{10}{3}x\)

\(\Rightarrow x=\frac{3}{250}\)

\(\frac{1}{2}\times\left(x-\frac{4}{5}\right)+\frac{3}{4}x=\frac{5}{12}\)

\(\Leftrightarrow\frac{1}{2}x-\frac{2}{5}+\frac{3}{4}x=\frac{5}{12}\)

\(\Leftrightarrow\frac{1}{2}x+\frac{3}{4}x=\frac{5}{12}+\frac{2}{5}\)

\(\Leftrightarrow\frac{5}{4}x=\frac{49}{60}\)

\(\Leftrightarrow x=\frac{49}{75}\)

Vậy \(x=\frac{49}{75}\)

bạn ơi sao lại có 2/5 vâỵ

phá ngoặc  \(\frac{1}{2}\times\frac{4}{5}=\frac{4}{10}=\frac{2}{5}\)  nhé bạn

\(B=\dfrac{3}{2}-\dfrac{2}{21}-\left\{\dfrac{7}{12}-\left[\dfrac{15}{21}-\left(\dfrac{1}{3}-\dfrac{5}{4}\right)-\left(\dfrac{2}{7}+\dfrac{1}{3}\right)\right]\right\}\)

\(B=\dfrac{3}{2}-\dfrac{2}{21}-\left\{\dfrac{7}{12}-\left[\dfrac{15}{21}-\left(-\dfrac{11}{12}\right)-\dfrac{13}{21}\right]\right\}\)

\(B=\dfrac{3}{2}-\dfrac{2}{21}-\left\{\dfrac{7}{12}-\dfrac{85}{84}\right\}\)

\(B=\dfrac{3}{2}-\dfrac{2}{21}-\left(-\dfrac{3}{7}\right)\)

\(B=\dfrac{11}{6}\)

\(=\dfrac{3}{2}-\dfrac{2}{21}-\dfrac{7}{12}+\left[\dfrac{15}{21}-\dfrac{1}{3}+\dfrac{5}{4}-\dfrac{2}{7}-\dfrac{1}{3}\right]\)

=11/12-2/21+5/7-2/3+5/4-2/7

=11/12-2/3+5/4-2/21+3/7

=11/12-8/12+15/12-2/21+9/21

=18/12+7/21

=3/2+1/3

=9/6+2/6=11/6