Đáp án:
$\begin{array}{l}
a)A = 3\left( {x - 1} \right) - 3x\left( {5 + x} \right) - \dfrac{2}{3}\\
= 3x - 3 - 15x - 3{x^2} - \dfrac{2}{3}\\
= - 3{x^2} - 12x - \dfrac{{11}}{3}\\
= - 3.{\left( {\dfrac{{ - 1}}{3}} \right)^2} - 12.\left( {\dfrac{{ - 1}}{3}} \right) - \dfrac{{11}}{3}\\
= \dfrac{{ - 1}}{3} + 4 - \dfrac{{11}}{3}\\
= 0\\
b)B = 3.\left( {1 - 2x} \right) - 2\left( {2x - 1} \right) - \dfrac{2}{5}\\
= 3 - 6x - 4x + 2 - \dfrac{2}{5}\\
= 5 - \dfrac{2}{5} - 10x\\
= \dfrac{{23}}{5} - 10x\\
= \dfrac{{23}}{5} - 10.\dfrac{{ - 1}}{5}\\
= \dfrac{{23}}{5} + 2\\
= \dfrac{{33}}{5}\\
c)C = - \left( {x - 4} \right) + 2\left( {3 - x} \right) - 5x\\
= - x + 4 + 6 - 2x - 5x\\
= 10 - 8x\\
= 10 - 8.\left( { - 1} \right)\\
= 18
\end{array}$
