Bài 5:
Ta có:
\(\dfrac{{{R_1}}}{{{R_2}}} = \dfrac{{{l_1}}}{{{l_2}}}.\dfrac{{{S_2}}}{{{S_1}}} \Rightarrow \dfrac{{120}}{{40}} = \dfrac{{100}}{{50}}.\dfrac{{{S_2}}}{{0,2}} \Rightarrow {S_2} = 0,3m{m^2}\)
Bài 7:
Ta có:
\(\begin{array}{l}
{R_1} = {R_2} \Rightarrow \dfrac{{{l_1}}}{{{S_1}}} = \dfrac{{{l_2}}}{{{S_2}}} \Rightarrow \dfrac{{{l_1}}}{{d_1^2}} = \dfrac{{{l_2}}}{{d_2^2}}\\
\Rightarrow \dfrac{{2,88}}{{0,{6^2}}} = \dfrac{{{l_2}}}{{0,{4^2}}} \Rightarrow {l_2} = 1,28m
\end{array}\)
