Đáp án:
$\begin{array}{l}
\frac{2}{{11.13}} + \frac{2}{{13.15}} + ... + \frac{2}{{19.21}}\\
= \frac{{13 - 11}}{{11.13}} + \frac{{15 - 13}}{{13.15}} + ... + \frac{{21 - 19}}{{19.21}}\\
= \frac{1}{{11}} - \frac{1}{{13}} + \frac{1}{{13}} - \frac{1}{{15}} + ... + \frac{1}{{19}} - \frac{1}{{21}}\\
= \frac{1}{{11}} - \frac{1}{{21}} = \frac{{10}}{{231}}\\
\Rightarrow \left( {\frac{2}{{11.13}} + \frac{2}{{13.15}} + ... + \frac{2}{{19.21}}} \right).462\\
- \left[ {2,04:\left( {x + 1,05} \right)} \right]:0,12 = 19\\
\Rightarrow \frac{{10}}{{231}}.462 - \left[ {2,04:\left( {x + 1,05} \right)} \right]:0,12 = 19\\
\Rightarrow \left[ {2,04:\left( {x + 1,05} \right)} \right]:0,12 = \frac{{10}}{{231}}.2.231 - 19\\
\Rightarrow \left[ {2,04:\left( {x + 1,05} \right)} \right]:0,12 = 20 - 19 = 1\\
\Rightarrow \left[ {2,04:\left( {x + 1,05} \right)} \right] = 1.0,12 = 0,12\\
\Rightarrow x + 1,05 = 2,04:0,12\\
\Rightarrow x + 1,05 = 17\\
\Rightarrow x = 17 - 1,05\\
\Rightarrow x = 15,95
\end{array}$
