Question

From the top of a building 60 m high, the angles of depression of the top and bottom of a tower are observed to be ${30}^{\circ }$ and ${60}^{\circ }$. Find the height of the tower.

• A. 80 m
• B. 90 m
• C. 70 m
• D. 40 m

Answer 1

The correct option is D

40 m

Let AB be the building and CD be the tower such that $\angle BDE={30}^{\circ },\angle BCA={60}^{\circ }$ and AB = 60 m

Let CA = DE = x metres.



From right $\Delta CAB$, we have

$\frac{CA}{AB}=cot{60}^{\circ }=\frac{1}{\sqrt{3}}$

$\Rightarrow \frac{x}{60}=\frac{1}{\sqrt{3}}\Rightarrow x=60\times \frac{1}{\sqrt{3}}$

$\Rightarrow x=60\times \frac{1}{\sqrt{3}}\times \frac{\sqrt{3}}{\sqrt{3}}=20\sqrt{3}$

$\Rightarrow CA=DE=20\sqrt{3}$

From right $\Delta BED$, we have

$\frac{BE}{DE}=tan{30}^{\circ }=\frac{1}{\sqrt{3}m}\Rightarrow \frac{BE}{20\sqrt{3}}=\frac{1}{\sqrt{3}}$ [using (i)]

$\Rightarrow BE=20\sqrt{3}\times \frac{1}{\sqrt{3}}m=20m$

$\therefore CD=AE=AB-BE=60m-20m-40m$

Hence the height of the tower is 40 m.