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Q9:
The angle of elevation of the top of a building from the foot of the tower is $30°$ and the angle of elevation of the top of the tower from the foot of the building is $60°$. If the tower is $50$ m high, find the height of the building.
Solution :
Given:
- Height of the tower ($CD$) = $50$ m.
- Angle of elevation of the top of the building ($AB$) from the foot of the tower ($D$) = $30^\circ$.
- Angle of elevation of the top of the tower ($CD$) from the foot of the building ($B$) = $60^\circ$.
To find:
The height of the building ($AB = h$).
Step 1: Define variables and identify triangles.
Let the height of the building $AB = h$ meters.
Let the distance between the foot of the building ($B$) and the foot of the tower ($D$) be $BD = x$ meters.
We have two right-angled triangles: $\triangle ABD$ and $\triangle CDB$.
Step 2: Analyze $\triangle CDB$ to find the distance $x$.
In $\triangle CDB$, the angle of elevation is $\angle CBD = 60^\circ$.
Using the trigonometric ratio tangent: $\tan(\theta) = \frac{\text{Opposite}}{\text{Adjacent}}$
$\tan(60^\circ) = \frac{CD}{BD}$
Since $\tan(60^\circ) = \sqrt{3}$ [Trigonometric table value]:
$\sqrt{3} = \frac{50}{x}$
$x = \frac{50}{\sqrt{3}}$ --- (Equation 1)
Step 3: Analyze $\triangle ABD$ to find the height $h$.
In $\triangle ABD$, the angle of elevation is $\angle ADB = 30^\circ$.
Using the trigonometric ratio tangent:
$\tan(30^\circ) = \frac{AB}{BD}$
Since $\tan(30^\circ) = \frac{1}{\sqrt{3}}$ [Trigonometric table value]:
$\frac{1}{\sqrt{3}} = \frac{h}{x}$
$h = \frac{x}{\sqrt{3}}$ --- (Equation 2)
Step 4: Substitute Equation 1 into Equation 2.
$h = \frac{\frac{50}{\sqrt{3}}}{\sqrt{3}}$
$h = \frac{50}{\sqrt{3} \times \sqrt{3}}$
$h = \frac{50}{3}$
$h = 16.666...$ meters
Final Answer: The height of the building is $\frac{50}{3}$ m or approximately $16.67$ m.
More Questions from Class 10 Mathematics Applications of Trigonometry EXERCISE 9.1
- Q1: A circus artist is climbing a $20$ m long rope, which is tightly stretched and tied from the top of a vertical pole to the ground. Find the height of the pole, if the angle made by the rope with the ground level is $30°$ (see Fig. 9.11).
- Q10: Two poles of equal heights are standing opposite each other on either side of the road, which is $80$ m wide. From a point between them on the road, the angles of elevation of the top of the poles are $60°$ and $30°$, respectively. Find the height of the poles and the distances of the point from the poles.
- Q11: A TV tower stands vertically on a bank of a canal. From a point on the other bank directly opposite the tower, the angle of elevation of the top of the tower is $60°$. From another point $20$ m away from this point on the line joing this point to the foot of the tower, the angle of elevation of the top of the tower is $30°$ (see Fig. 9.12). Find the height of the tower and the width of the canal.
- Q12: From the top of a $7$ m high building, the angle of elevation of the top of a cable tower is $60°$ and the angle of depression of its foot is $45°$. Determine the height of the tower.
- Q13: As observed from the top of a $75$ m high lighthouse from the sea-level, the angles of depression of two ships are $30°$ and $45°$. If one ship is exactly behind the other on the same side of the lighthouse, find the distance between the two ships.
- Q14: A $1.2$ m tall girl spots a balloon moving with the wind in a horizontal line at a height of $88.2$ m from the ground. The angle of elevation of the balloon from the eyes of the girl at any instant is $60°$. After some time, the angle of elevation reduces to $30°$ (see Fig. 9.13). Find the distance travelled by the balloon during the interval.
- Q15: A straight highway leads to the foot of a tower. A man standing at the top of the tower observes a car at an angle of depression of $30°$, which is approaching the foot of the tower with a uniform speed. Six seconds later, the angle of depression of the car is found to be $60°$. Find the time taken by the car to reach the foot of the tower from this point.
- Q2: A tree breaks due to storm and the broken part bends so that the top of the tree touches the ground making an angle $30°$ with it. The distance between the foot of the tree to the point where the top touches the ground is $8$ m. Find the height of the tree.
- Q3: A contractor plans to install two slides for the children to play in a park. For the children below the age of $5$ years, she prefers to have a slide whose top is at a height of $1.5$ m, and is inclined at an angle of $30°$ to the ground, whereas for elder children, she wants to have a steep slide at a height of $3$m, and inclined at an angle of $60°$ to the ground. What should be the length of the slide in each case?
- Q4: The angle of elevation of the top of a tower from a point on the ground, which is $30$ m away from the foot of the tower, is $30°$. Find the height of the tower.
- Q5: A kite is flying at a height of $60$ m above the ground. The string attached to the kite is temporarily tied to a point on the ground. The inclination of the string with the ground is $60°$. Find the length of the string, assuming that there is no slack in the string.
- Q6: A $1.5$ m tall boy is standing at some distance from a $30$ m tall building. The angle of elevation from his eyes to the top of the building increases from $30°$ to $60°$ as he walks towards the building. Find the distance he walked towards the building.
- Q7: From a point on the ground, the angles of elevation of the bottom and the top of a transmission tower fixed at the top of a $20$ m high building are $45°$ and $60°$ respectively. Find the height of the tower.
- Q8: A statue, $1.6$ m tall, stands on the top of a pedestal. From a point on the ground, the angle of elevation of the top of the statue is $60°$ and from the same point the angle of elevation of the top of the pedestal is $45°$. Find the height of the pedestal.
CBSE Solutions for Class 10 Mathematics Applications of Trigonometry
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