Đáp án:
$\begin{array}{l}
a)Do:\frac{3}{{2008}} < 1\\
\Rightarrow \frac{3}{{2008}} < \frac{{3 + 1}}{{2008 + 1}}\\
\Rightarrow \frac{3}{{2008}} < \frac{4}{{2009}}\\
b) - \frac{9}{{41}} = - \frac{{9.101}}{{41.101}} = - \frac{{909}}{{4141}}\\
c)\frac{{18}}{{37}} > \frac{{18 - 1}}{{37}}\\
\Rightarrow \frac{{18}}{{37}} > \frac{{17}}{{37}}\\
Mà:\frac{{17}}{{37}} > \frac{{17}}{{40}}\\
\Rightarrow \frac{{18}}{{37}} > \frac{{17}}{{40}}\\
d)\frac{{21}}{{90}} = \frac{{7.3}}{{30.3}} = \frac{7}{{30}}\\
\frac{{ - 75}}{{ - 150}} = \frac{{ - 75}}{{ - 75.2}} = \frac{1}{2}\\
Do:\frac{7}{{30}} < \frac{1}{2}\\
\Rightarrow \frac{{21}}{{90}} < \frac{{ - 75}}{{ - 150}}
\end{array}$
