Question

The angle of elevation of a cloud from a point 60 m above a lake is ${30}^{\circ }$ and the angle of depression of the reflection of cloud in the lake is ${60}^{\circ }$. Find the height of the cloud.

• A. 820 metres
• B. 120 metres
• C. 190 metres
• D. 320 metres.

Answer 1

The correct option is B

120 metres

Concept: 2 Marks
Application: 2 Marks

Let AB be the surface of the lake and let P be a point vertically above A such that AP = 60 m



Let C be the position of the cloud and let D be its reflection in the lake.

Draw $PQ\perp CD$, Then,

$\angle QPC={30}^{\circ },\angle QPD={60}^{\circ }$,

BQ = AP = 60 m

Let $CQ=xmetres$. Then,

$BD=BC=(x+60)m$

From right $\Delta PQC$, we have

$\frac{PQ}{CQ}=cot{30}^{\circ }=\sqrt{3}$

$\Rightarrow \frac{PQ}{xm}=\sqrt{3}\Rightarrow PQ=x\sqrt{3}m$

From right $\Delta PQD$, we have

$\frac{PQ}{QD}=cot{60}^{\circ }=\frac{1}{\sqrt{3}}$

$\Rightarrow \frac{PQ}{(x+60+60)m}=\frac{1}{\sqrt{3}}\Rightarrow PQ=\frac{(x+120)}{\sqrt{3}}m$

Equating the values of PQ from (i) and (ii), we get

$x\sqrt{3}=\frac{(x+120)}{\sqrt{3}}m$

$\Rightarrow 3x=x+120\Rightarrow 2x=120\Rightarrow x=60$

$\therefore$ height of the cloud from the surface of the lake

$=BC=(60+x)m=(60+60)m=120m$

Hence, the height of the cloud from the surface of the lake is 120 metres.