Đáp án:
$\begin{array}{l}
C1)\\
a)\dfrac{{ - 11}}{6} < - 1\\
\dfrac{8}{{ - 9}} > - 1\\
\Leftrightarrow \dfrac{{ - 11}}{6} < \dfrac{8}{{ - 9}}\\
b)\dfrac{2}{7} = \dfrac{{10}}{{35}}\\
\dfrac{1}{5} = \dfrac{7}{{35}} < \dfrac{{10}}{{35}}\\
\Leftrightarrow \dfrac{2}{7} > \dfrac{1}{5}\\
c)\dfrac{{ - 30}}{{55}} = \dfrac{{ - 6}}{{11}} = \dfrac{6}{{ - 11}}\\
d)\dfrac{{2009}}{{2010}} < 1 < \dfrac{{2010}}{{2009}}\\
C2)\\
a)\dfrac{{ - 1}}{{21}} + \dfrac{{ - 1}}{{14}} = \dfrac{{ - 2 - 3}}{{42}} = \dfrac{{ - 5}}{{42}}\\
b)4,5 - \left( {\dfrac{{ - 7}}{5}} \right) = 4,5 + 1,4 = 5,9\\
c)\left( {\dfrac{{ - 24}}{{11}}} \right) + \left( {\dfrac{{ - 19}}{{13}}} \right) + \dfrac{2}{{11}} + \left( {\dfrac{{ - 20}}{{13}}} \right)\\
= \left( {\dfrac{{ - 24}}{{11}} + \dfrac{2}{{11}}} \right) + \left( {\dfrac{{ - 19}}{{13}} - \dfrac{{20}}{{13}}} \right)\\
= \dfrac{{ - 22}}{{11}} - \dfrac{{39}}{{13}}\\
= - 2 - 3\\
= - 5\\
d)\left( {\dfrac{{ - 25}}{{13}}} \right) + \left( { - \dfrac{9}{{17}}} \right) + \dfrac{{12}}{{13}} + \left( {\dfrac{{ - 25}}{{17}}} \right)\\
= \left( {\dfrac{{ - 25}}{{13}} + \dfrac{{12}}{{13}}} \right) + \left( { - \dfrac{9}{{17}} - \dfrac{{25}}{{17}}} \right)\\
= \dfrac{{ - 13}}{{13}} + \dfrac{{ - 34}}{{17}}\\
= - 1 - 2\\
= - 3\\
C5)\\
A = \dfrac{1}{{1.3}} + \dfrac{1}{{3.5}} + \dfrac{1}{{5.7}} + ... + \dfrac{1}{{19.21}}\\
= \dfrac{1}{2}.\left( {\dfrac{2}{{1.3}} + \dfrac{2}{{3.5}} + \dfrac{2}{{5.7}} + ... + \dfrac{2}{{19.21}}} \right)\\
= \dfrac{1}{2}.\left( {1 - \dfrac{1}{3} + \dfrac{1}{3} - \dfrac{1}{5} + \dfrac{1}{5} - \dfrac{1}{7} + ... + \dfrac{1}{{19}} - \dfrac{1}{{21}}} \right)\\
= \dfrac{1}{2}.\left( {1 - \dfrac{1}{{21}}} \right)\\
= \dfrac{1}{2}.\dfrac{{20}}{{21}}\\
= \dfrac{{10}}{{21}}
\end{array}$
