Đáp án D

\({2.6^{f\left( x \right)}} + \left( {{f^2}\left( x \right) - 1} \right){.9^{f\left( x \right)}} - {3.4^{f\left( x \right)}}.m \ge \left( {2{m^2} + 2m} \right){.2^{2f\left( x \right)}},\forall x \in \mathbb{R}\)

\( \Leftrightarrow \left( {{f^2}\left( x \right) - 1} \right){.9^{f\left( x \right)}} + {2.6^{f\left( x \right)}} - \left( {2{m^2} + 5m} \right){.4^{f\left( x \right)}} \ge 0,\forall x \in \mathbb{R}\)

\( \Leftrightarrow \left( {{f^2}\left( x \right) - 1} \right).{\left( {\frac{9}{4}} \right)^{f\left( x \right)}} + 2.{\left( {\frac{3}{2}} \right)^{f\left( x \right)}} - 2{m^2} - 5m \ge 0,\forall x \in \mathbb{R}\)

\( \Leftrightarrow 2{m^2} + 5m \le \left( {{f^2}\left( x \right) - 1} \right).{\left( {\frac{9}{4}} \right)^{f\left( x \right)}} + 2.{\left( {\frac{3}{2}} \right)^{f\left( x \right)}},\forall x \in \mathbb{R}\) (1)

Đặt \(t = f\left( x \right) \ge 1,{\rm{ }}\forall {\rm{x}} \in \mathbb{R}\). (1) thành: \(2{m^2} + 5m \le \left( {{t^2} - 1} \right){\left( {\frac{9}{4}} \right)^t} + 2{\left( {\frac{3}{2}} \right)^t},\forall t \in \left[ {1; + \infty } \right)\)

Đặt \(g\left( t \right) = \left( {{t^2} - 1} \right).{\left( {\frac{9}{4}} \right)^t} + 2{\left( {\frac{3}{2}} \right)^t},\forall t \in \left[ {1; + \infty } \right)\)

\( \Rightarrow g'\left( t \right) = 2t.{\left( {\frac{9}{4}} \right)^t} + \left( {{t^2} - 1} \right).{\left( {\frac{9}{4}} \right)^t}\ln \frac{9}{4} + 2.{\left( {\frac{3}{2}} \right)^t}\ln \frac{3}{2} > 0,\forall t \in \left[ {1; + \infty } \right)\)

Suy ra \(g\left( t \right) \ge g\left( 1 \right) = 3,\forall t \in \left[ {1; + \infty } \right)\).

Yêu cầu bài toán \( \Leftrightarrow 2{m^2} + 5m \le 3 \Leftrightarrow - 3 \le m \le \frac{1}{2}\).

Do \(m \in \mathbb{Z} \Rightarrow m \in \left\{ { - 3; - 2; - 1;0} \right\}\) nên có 4 giá trị nguyên thỏa mãn.

Hình hộp chữ nhật ABCD.A'B'C'D' có AB=a và AD=3a