Tìm số nguyên x biết.

* Hướng dẫn giải

a) 

\(\begin{array}{l}
5 - 2x =  - 21\\
\,\,\, - 2x =  - 21 - 5\\
\,\,\,\,\,\,\,\,\,\,x =  - 26:\left( { - 2} \right)\\
\,\,\,\,\,\,\,\,\,\,x = 13
\end{array}\)

b) \(\frac{{ - 3}}{x} = \frac{3}{5} + \frac{{ - 9}}{{20}}\)

\(\begin{array}{l}
\frac{{ - 3}}{x} = \frac{{12}}{{20}} + \frac{{ - 9}}{{20}} = \frac{{12 + ( - 9)}}{{20}}\\
\frac{{ - 3}}{x} = \frac{3}{{20}}\\
x = \frac{{20.\left( { - 3} \right)}}{3} =  - 20
\end{array}\)