Đáp án:
D
Giải thích các bước giải:
\[\begin{array}{l}
i = 0,7\left( {mm} \right)\\
{x_{t4}} = x{'_t}\\
\Rightarrow 3,5i = \left( {k + 0,5} \right)i' = \left( {k + 0,5} \right)\frac{{D\lambda '}}{a}\\
\Rightarrow 3,5.0,{7.10^{ - 3}} = \left( {k + 0,5} \right)\frac{{1,4.\lambda '}}{{{{10}^{ - 3}}}}\\
\Rightarrow \lambda ' = \frac{{1,{{75.10}^{ - 6}}}}{{k + 0,5}}\\
0,{38.10^{ - 6}} < \lambda ' < 0,{76.10^{ - 6}}\\
\Rightarrow 0,{38.10^{ - 6}} < \frac{{1,{{75.10}^{ - 6}}}}{{k + 0,5}} < 0,{76.10^{ - 6}}\\
\Rightarrow 4,1 > k > 1,8\\
\Rightarrow k = 2,3,4\\
k = 2 \Rightarrow \lambda ' = 0,{7.10^{ - 6}}\left( m \right)\\
k = 3 \Rightarrow \lambda ' = 0,{5.10^{ - 6}}\left( m \right)\\
k = 4 \Rightarrow \lambda ' = 0,{389.10^{ - 6}}\left( m \right)
\end{array}\]
