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:

\(BC={{r}_{C}}-{{r}_{B}}=157,3-(30+49,73)=77,57\approx 78m\)