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C

- Hướng dẫn giải

Ban đầu ta có \(\Delta P = R\frac{{{P^2}}}{U} \to \frac{{\Delta {P_1}}}{{\Delta {P_2}}} = \frac{{P_1^2}}{{P_2^2}}.\frac{{U_2^2}}{{U_1^2}} \leftrightarrow \frac{{\Delta {P_1}}}{{{P_1}}} = \frac{{\Delta {P_2}}}{{{P_2}}}.\frac{{{P_1}}}{{{P_2}}}.\frac{{U_1^2}}{{U_2^2}}\)

\(\begin{array}{l}
H = 1 - \frac{{\Delta P}}{P} \to \frac{{\Delta P}}{P} = 1 - H \to 1 - {H_1} = \left( {1 - {H_2}} \right)\frac{{{P_1}}}{{{P_2}}}.\frac{{U_2^2}}{{U_1^2}}\\
 \leftrightarrow \frac{{1 - {H_1}}}{{1 - {H_2}}} = \frac{{{P_1}}}{{{P_2}}}.\frac{{U_2^2}}{{U_1^2}}
\end{array}\)

 \(\begin{array}{l}
{P_1} = \frac{{{P_{tt}}}}{{{H_1}}};{P_2} = \frac{{{P_{tt}}}}{{{H_2}}}\\
 \to \frac{{{P_1}}}{{{P_2}}} = \frac{{{H_2}}}{{{H_1}}} \to \frac{{1 - {H_1}}}{{1 - {H_2}}} = \frac{{{H_2}}}{{{H_1}}}\frac{{U_2^2}}{{U_1^2}}\\
 \to \frac{{1 - 0,8}}{{1 - {H_2}}} = \frac{{{H_2}}}{{0,8}}.4\\
 \to {H_2} = 0,9582
\end{array}\)