Đáp án:
$\begin{array}{l}
11)\\
a){\left( {x - 3} \right)^2} - 4 = 0\\
\Leftrightarrow {\left( {x - 3} \right)^2} = 4\\
\Leftrightarrow \left[ \begin{array}{l}
x - 3 = 2\\
x - 3 = - 2
\end{array} \right.\\
\Leftrightarrow \left[ \begin{array}{l}
x = 5\\
x = 1
\end{array} \right.\\
Vậy\,x = 1;x = 5\\
b){x^2} - 2x = 24\\
\Leftrightarrow {x^2} - 2x - 24 = 0\\
\Leftrightarrow {x^2} - 6x + 4x - 24 = 0\\
\Leftrightarrow \left( {x - 6} \right)\left( {x + 4} \right) = 0\\
\Leftrightarrow \left[ \begin{array}{l}
x - 6 = 0\\
x + 4 = 0
\end{array} \right.\\
\Leftrightarrow \left[ \begin{array}{l}
x = 6\\
x = - 4
\end{array} \right.\\
Vậy\,x = - 4;x = 6\\
c){\left( {2x - 1} \right)^2} + {\left( {x + 3} \right)^2} - 5\left( {x + 7} \right)\left( {x - 7} \right) = 0\\
\Leftrightarrow 4{x^2} - 4x + 1 + {x^2} + 6x + 9 - 5\left( {{x^2} - 49} \right) = 0\\
\Leftrightarrow 5{x^2} + 2x + 10 - 5{x^2} + 245 = 0\\
\Leftrightarrow 2x = - 255\\
\Leftrightarrow x = \dfrac{{ - 255}}{2}\\
Vậy\,x = \dfrac{{ - 255}}{2}\\
d)25{x^2} - 9 = 0\\
\Leftrightarrow {x^2} = \dfrac{9}{{25}}\\
\Leftrightarrow x = \dfrac{3}{5};x = - \dfrac{3}{5}\\
Vậy\,x = \dfrac{3}{5};x = - \dfrac{3}{5}\\
e){\left( {x + 4} \right)^2} - \left( {x + 1} \right)\left( {x - 1} \right) = 16\\
\Leftrightarrow {x^2} + 8x + 16 - {x^2} + 1 = 16\\
\Leftrightarrow 8x = - 1\\
\Leftrightarrow x = - \dfrac{1}{8}\\
Vậy\,x = - \dfrac{1}{8}\\
h){\left( {x - 3} \right)^2} + x\left( {x + 4} \right) - 2{\left( {x + 1} \right)^2} = 3\\
\Leftrightarrow {x^2} - 6x + 9 + {x^2} + 4x - 2{x^2} - 4x - 2 = 3\\
\Leftrightarrow - 6x = - 4\\
\Leftrightarrow x = \dfrac{2}{3}\\
Vậy\,x = \dfrac{2}{3}\\
B12)\\
a)A = {\left( {x - 10} \right)^2} - x\left( {x + 80} \right)\\
= {x^2} - 20x + 100 - {x^2} - 80x\\
= - 100x + 100\\
= 100\left( {1 - x} \right)\\
= 100.\left( {1 - 0,98} \right)\\
= 100.0,02\\
= 2\\
B = {\left( {2x + 9} \right)^2} - x\left( {4x + 3} \right)\\
= 4{x^2} + 36x + 81 - 4{x^2} - 3x\\
= 33x + 81\\
= 33.\left( { - 16,2} \right) + 81\\
= - 453,6\\
C = 4{x^2} - 28x + 49\\
= {\left( {2x - 7} \right)^2}\\
= {\left( {2.4 - 7} \right)^2}\\
= {1^2} = 1
\end{array}$
