A tower $T_1$ of height $60 \mathrm{~m}$ is located exactly opposite to a tower $T_2$ of height $80…

A tower $T_1$ of height $60 \mathrm{~m}$ is located exactly opposite to a tower $T_2$ of height $80 \mathrm{~m}$ on a straight road. From the top of $T_1$, if the angle of depression of the foot of $T_2$ is twice the angle of elevation of the top of $T_2$, then the width (in $m$ ) of the road between the feet of the towers $T_1$ and $T_2$ is
  1. $20 \sqrt{2}$
  2. $10 \sqrt{2}$
  3. $10 \sqrt{3}$
  4. $20 \sqrt{3}$

Solution

$ \text { Let the distance between } T_1 \text { and } T_2 \text { be } x $
From the figure $E A=60 \mathrm{~m}\left(T_1\right)$ and $D B=80 \mathrm{~m}\left(T_2\right)$ $\angle D E C=\theta$ and $\angle B E C=2 \theta$ Now in $\triangle D E C$, $ \tan \theta=\frac{D C}{A B}=\frac{20}{x} $ and in $\triangle B E C$, $ \tan 2 \theta=\frac{B C}{C E}=\frac{60}{x} $ We know that $ \tan 2 \theta=\frac{2 \tan \theta}{1-(\tan \theta)^2} $ $ \begin{aligned} &\Rightarrow \frac{60}{x}=\frac{2\left(\frac{20}{x}\right)}{1-\left(\frac{20}{x}\right)^2} \\ &\Rightarrow x^2=1200 \Rightarrow x=20 \sqrt{3} \end{aligned} $

Asked in: JEE Main 2018 (15 Apr Shift 2 Online)

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