Đáp án:

 Bài 2)
$1)
x=4\\
2)
x=\dfrac{-3}{5}\\
3)
x=\dfrac{25}{2}\\
4)
 x=6\\
5)
 {\left[\begin{aligned}x=\dfrac{7}{12}\\x=-\dfrac{1}{4}\end{aligned}\right.}\\
6)
 {\left[\begin{aligned}x=-\dfrac{1}{2}\\ x= \dfrac{1}{3}\end{aligned}\right.}\\
7)
x=\dfrac{5}{3}\\
8)
 {\left[\begin{aligned}x=\dfrac{5}{5}=1\\x=-\dfrac{7}{5}\end{aligned}\right.}\\
9)
 x=\dfrac{7}{6}$

Giải thích các bước giải:

 Bài 2)
$1)
2x-21=-13\Leftrightarrow 2x=-13+21=8\Leftrightarrow x=4\\
2)
0,25-\dfrac{1}{4}:x=\dfrac{2}{3}\Leftrightarrow \dfrac{1}{4}-\dfrac{1}{4}:x=\dfrac{2}{3}\\
\Leftrightarrow \dfrac{1}{4}.\dfrac{1}{x}=\dfrac{1}{4}-\dfrac{2}{3}\\
\Leftrightarrow \dfrac{1}{4}.\dfrac{1}{x}=\dfrac{3}{12}-\dfrac{8}{12}\\
\Leftrightarrow \dfrac{1}{4}.\dfrac{1}{x}=\dfrac{-5}{12}\\
\Leftrightarrow \dfrac{1}{x}=\dfrac{-5}{12}.4=\dfrac{-5}{3}\\
\Leftrightarrow x=\dfrac{-3}{5}\\
3)
5\dfrac{2}{7}-\dfrac{2}{5}x=\dfrac{2}{7}\\
\Leftrightarrow \dfrac{2}{5}x=\dfrac{37}{7}-\dfrac{2}{7}=\dfrac{35}{7}=5\\
\Leftrightarrow x=5.\dfrac{5}{2}=\dfrac{25}{2}\\
4)
\dfrac{x-2}{10}=\dfrac{2}{5}\Leftrightarrow x-2=\dfrac{2}{5}.10=4\\
\Leftrightarrow x=6\\
5)
\left |2x-\dfrac{1}{3}  \right |+\dfrac{1}{2}=\dfrac{4}{3}\\
\Leftrightarrow \left |2x-\dfrac{1}{3}  \right |=\dfrac{4}{3}-\dfrac{1}{2}\\
\Leftrightarrow \left |2x-\dfrac{1}{3}  \right |=\dfrac{8}{6}-\dfrac{3}{6}\\
\Leftrightarrow \left |2x-\dfrac{1}{3}  \right |=\dfrac{5}{6}\\
\Leftrightarrow {\left[\begin{aligned}2x-\dfrac{1}{3}=\dfrac{5}{6}\\2x-\dfrac{1}{3}=-\dfrac{5}{6}\end{aligned}\right.}\\
\Leftrightarrow {\left[\begin{aligned}2x=\dfrac{5}{6}+\dfrac{1}{3}\\2x=-\dfrac{5}{6}+\dfrac{1}{3}\end{aligned}\right.}\\
\Leftrightarrow {\left[\begin{aligned}2x=\dfrac{5}{6}+\dfrac{2}{6}\\2x=-\dfrac{5}{6}+\dfrac{2}{6}\end{aligned}\right.}\\
\Leftrightarrow {\left[\begin{aligned}2x=\dfrac{7}{6}\\2x=-\dfrac{1}{2}\end{aligned}\right.}\\
\Leftrightarrow {\left[\begin{aligned}x=\dfrac{7}{12}\\x=-\dfrac{1}{4}\end{aligned}\right.}\\
6)
\left ( x+\dfrac{1}{2} \right ).\left ( \dfrac{2}{3}-2x \right) =0\\
\Leftrightarrow {\left[\begin{aligned}x+\dfrac{1}{2}=0\\  \dfrac{2}{3}-2x=0\end{aligned}\right.}\\
\Leftrightarrow {\left[\begin{aligned}x=-\dfrac{1}{2}\\ 2x= \dfrac{2}{3}\end{aligned}\right.}\\
\Leftrightarrow {\left[\begin{aligned}x=-\dfrac{1}{2}\\ x= \dfrac{1}{3}\end{aligned}\right.}\\
7)
x-\dfrac{25}{100}x=\dfrac{125}{100}\\
\Leftrightarrow x-\dfrac{1}{4}x=\dfrac{5}{4}\\
\Leftrightarrow \dfrac{4}{4}x-\dfrac{1}{4}x=\dfrac{5}{4}\\
\Leftrightarrow \dfrac{3}{4}x=\dfrac{5}{4}\\
\Leftrightarrow x=\dfrac{5}{4}.\dfrac{4}{3}=\dfrac{5}{3}\\
8)
\left ( x+\dfrac{1}{5} \right )^2-\dfrac{17}{25}=\dfrac{19}{25}\\
\Leftrightarrow \left ( x+\dfrac{1}{5} \right )^2=\dfrac{17}{25}+\dfrac{19}{25}\\
\Leftrightarrow \left ( x+\dfrac{1}{5} \right )^2=\dfrac{36}{25}\\
\Leftrightarrow {\left[\begin{aligned}x+\dfrac{1}{5}=\dfrac{6}{5}\\x+\dfrac{1}{5}=-\dfrac{6}{5}\end{aligned}\right.}\\
\Leftrightarrow {\left[\begin{aligned}x=\dfrac{6}{5}-\dfrac{1}{5}\\x=-\dfrac{6}{5}-\dfrac{1}{5}\end{aligned}\right.}\\
\Leftrightarrow {\left[\begin{aligned}x=\dfrac{5}{5}=1\\x=-\dfrac{7}{5}\end{aligned}\right.}\\
9)
1\dfrac{5}{27}-\left ( x-\dfrac{1}{2} \right )^3=\dfrac{24}{27}\\
\Leftrightarrow \left ( x-\dfrac{1}{2} \right )^3=\dfrac{32}{27}-\dfrac{24}{27}\\
\Leftrightarrow \left ( x-\dfrac{1}{2} \right )^3=\dfrac{8}{27}\\
\Leftrightarrow x-\dfrac{1}{2}=\dfrac{2}{3}\\
\Leftrightarrow x=\dfrac{2}{3}+\dfrac{1}{2}
\Leftrightarrow x=\dfrac{4}{6}+\dfrac{3}{6}
\Leftrightarrow x=\dfrac{7}{6}$