Đáp án:
$\begin{array}{l}
a)B\left( 1 \right) = {6.1^2} - 7.1 + 1 = 0\\
B\left( { - 2} \right) = 6.{\left( { - 2} \right)^2} - 7.\left( { - 2} \right) + 1 = 39\\
b)A\left( x \right) = 2{x^4} - 3{x^2} + 5x + 3 + 2x\\
= 2{x^4} - 3{x^2} + 7x + 3\\
c)A\left( x \right) + B\left( x \right)\\
= 2{x^4} - 3{x^2} + 7x + 3 + 6{x^2} - 7x + 1\\
= 2{x^4} + 3{x^2} + 4\\
A\left( x \right) - B\left( x \right)\\
= 2{x^4} - 3{x^2} + 7x + 3 - 6{x^2} + 7x - 1\\
= 2{x^4} - 9{x^2} + 14x + 2\\
d)P = A\left( x \right) + B\left( x \right)\\
= 2{x^4} + 3{x^2} + 4\\
= 2{\left( {{x^2}} \right)^2} + 3{x^2} + 4\\
Do:{x^2} \ge 0\\
\Leftrightarrow 2{\left( {{x^2}} \right)^2} + 3{x^2} \ge 0\\
\Leftrightarrow 2{x^4} + 3{x^2} + 4 \ge 4 > 0\\
Vậy\,P\left( x \right) > 0
\end{array}$
