Đáp án:

Ta có: 

\(\begin{array}{l}
AB = 4 + 8 + 4 = 16cm\\
NB = \sqrt {{8^2} + {{\left( {8 + 4} \right)}^2}}  = 4\sqrt {13} cm\\
NA = \sqrt {{4^2} + {8^2}}  = 4\sqrt 5 cm\\
{k_A} = \dfrac{{AB - 0}}{\lambda } = \dfrac{{16 - 0}}{{2,5}} = 6,4\\
{k_N} = \dfrac{{NB - NA}}{\lambda } = \dfrac{{4\sqrt {13}  - 4\sqrt 5 }}{{2,5}} = 2,1
\end{array}\)