Đáp án:
$\begin{array}{l}
B = {x^{10}} + {20^9} + {20^8} + ... + {20^2} + 20x + 20\\
= {x^{10}} + 20x + \left( {{{20}^9} + {{20}^8} + ... + {{20}^2} + 20} \right)\\
= {x^{10}} + 20x + A\\
A = {20^9} + {20^8} + .. + {20^2} + 20\\
\Rightarrow 20A = {20^{10}} + {20^9} + ... + {20^3} + {20^2}\\
\Rightarrow 20A - A = {20^{10}} - 20\\
\Rightarrow 19A = {20^{10}} - 20\\
\Rightarrow A = \dfrac{{{{20}^{10}} - 20}}{{19}}\\
\Rightarrow B = {x^{10}} + 20x + \dfrac{{{{20}^{10}} - 20}}{{19}}\\
x = - 19\\
\Rightarrow B = {\left( { - 19} \right)^{10}} + 20.\left( { - 19} \right) + \dfrac{{{{20}^{10}} - 20}}{{19}}\\
= {19^{10}} + \dfrac{{{{20}^{10}} - 20}}{{19}} - 380
\end{array}$
