Question : From the top of a lighthouse at a height of 20 metres above sea-level, the angle of depression of a ship is 30°. The distance of the ship from the foot of the lighthouse is:
Option 1: $20$ m
Option 2: $20 {\sqrt3}$ m
Option 3: $30$ m
Option 4: $30 {\sqrt3}$ m
Correct Answer: $20 {\sqrt3}$ m
Solution : Height of a lighthouse = 20 metres Let the distance of the ship from the foot of the lighthouse be $x$. The angle of depression of the ship from the house = $30°$ From figure, we can see that $\frac{20}{x}=\tan 30°$ So, the distance of the ship from the foot of the lighthouse = $20\cot 30°=20\sqrt{3}$ Hence, the correct answer is $20\sqrt{3}$ metres.
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Question : As observed from the top of a lighthouse, 45 m high above the sea level, the angle of depression of a ship, sailing directly towards it, changes from 30° to 45°. The distance travelled by ship during the period of observation is: (Your answer should be correct to one decimal place.)
Option 1: 32.9 m
Option 2: 33.4 m
Option 3: 36.9 m
Option 4: 24.8 m
Question : A person observes that the angle of elevation at the top of a pole of height 5 metres is 30°. Then the distance of the person from the pole is:
Option 1: $5\sqrt3$ metres
Option 2: $\frac{5}{\sqrt3}$ metres
Option 3: $\sqrt3$ metres
Option 4: $10\sqrt3$ metres
Question : A person observes that the angle of elevation of the top of a pole of height 15 metres is 30°. What is the distance (in metres) of the person from the pole?
Option 1: $15$
Option 2: $15\sqrt3$
Option 3: $\frac{15}{\sqrt3}$
Option 4: $30$
Question : From a point 12 m above the water level, the angle of elevation of the top of a hill is 60° and the angle of depression of the base of the hill is 30°. What is the height (in m) of the hill?
Option 1: $48 \sqrt{3}$
Option 2: $36$
Option 3: $36 \sqrt{3}$
Option 4: $48$
Question : From the peak of a hill 300 m high, the angle of depression of two sides of a bridge lying on the ground are $45°$ and $30°$ (both ends of the bridge are on the same side of the hill). Then the length of the bridge is:
Option 1: $300(\sqrt3 - 1)$ m
Option 2: $300(\sqrt3 + 1)$ m
Option 3: $300\sqrt3$ m
Option 4: $\frac{300}{\sqrt3}$ m
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