Đáp án:
$\begin{array}{l}
a)A = 4x - {\left( {x + 3} \right)^2} + \left( {x + 3} \right)\left( {x - 3} \right)\\
= 4x - \left( {{x^2} + 6x + 9} \right) + {x^2} - 9\\
= 4x - {x^2} - 6x - 9 + {x^2} - 9\\
= - 2x - 18\\
= - 2.\left( { - 2,4} \right) - 18\\
= 4,8 - 18\\
= 13,2\\
b)B = {\left( {3x + 4} \right)^2} - 10x - 9\left( {x - 4} \right)\left( {4 + x} \right)\\
= 9{x^2} + 24x + 16 - 10x - 9\left( {{x^2} - 16} \right)\\
= 9{x^2} + 14x + 16 - 9{x^2} + 144\\
= 14x + 160\\
= 14.\left( { - 0,1} \right) + 160\\
= - 1,4 + 160\\
= 159,6\\
c)\\
C = {\left( {x + 5} \right)^2} - 2\left( {x - 6} \right)\left( {x + 6} \right) + \left( {x + 2} \right)\left( {x - 4} \right)\\
= {x^2} + 10x + 25 - 2\left( {{x^2} - 36} \right) + {x^2} - 2x - 8\\
= 2{x^2} + 8x + 17 - 2{x^2} + 72\\
= 8x + 89\\
= 8.\dfrac{{ - 1}}{2} + 89\\
= 85
\end{array}$
