Đáp án:
$BC=78m$
Giải thích các bước giải:
$\begin{align}
& {{L}_{A}}=40dB;{{L}_{B}}=35,9dB;{{L}_{C}}=30dB; \\
& AB=30m;BC=? \\
\end{align}$
Ta có:
$\begin{align}
& {{L}_{B}}-{{L}_{A}}=10.\lg \left( \dfrac{r_{A}^{2}}{r_{B}^{2}} \right) \\
& \Leftrightarrow 35,9-40=10.\lg \left( \dfrac{r_{A}^{2}}{{{({{r}_{A}}+30)}^{2}}} \right) \\
& \Rightarrow {{r}_{A}}=49,73m \\
\end{align}$
mà:
$\begin{align}
& {{L}_{C}}-{{L}_{B}}=10.\lg \left( \dfrac{r_{B}^{2}}{r_{C}^{2}} \right) \\
& \Leftrightarrow 30-35,9=10.\lg \left( \dfrac{{{(30+49,73)}^{2}}}{{{({{r}_{C}})}^{2}}} \right) \\
& \Rightarrow {{r}_{C}}=157,3m \\
\end{align}$
khoảng cách BC:
\(BC={{r}_{C}}-{{r}_{B}}=157,3-(30+49,73)=77,57\approx 78m\)