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Question

The angles of elevation of the top of a cliff from two points on the ground, $200\text{ m}$ apart, and on opposite sides of the cliff, are $30^{\circ}$ and $45^{\circ}$. What is the height of the cliff?

This question was previously asked in
SSC CGL 2025 Tier 1 Question Paper (25-Sep-2025) (Shift 3)
The correct answer is
$100(\sqrt{3} - 1)\text{m}$

Cliff Height Calculation Using Angles of Elevation

This solution calculates the height of a cliff based on the angles of elevation observed from two ground points separated by $200\text{ m}$ on opposite sides of the cliff.

Steps to Find Cliff Height

  1. Define Variables:

    Let the cliff height be $h$. Let the distances from the cliff base to the observation points be $x$ and $y$. We are given $x + y = 200\text{ m}$.

  2. Apply Trigonometric Ratios:

    Using the angles of elevation:

    • For the point with $30^{\circ}$ angle: $\tan(30^{\circ}) = \frac{h}{x}$
    • For the point with $45^{\circ}$ angle: $\tan(45^{\circ}) = \frac{h}{y}$
  3. Express Distances ($x$, $y$) in Terms of Height ($h$):
    • $x = \frac{h}{\tan(30^{\circ})} = h \cot(30^{\circ}) = h\sqrt{3}$
    • $y = \frac{h}{\tan(45^{\circ})} = h \cot(45^{\circ}) = h(1) = h$
  4. Combine Equations:

    Substitute $x$ and $y$ into the distance equation $x + y = 200$:

    $ h\sqrt{3} + h = 200 $

  5. Solve for $h$:

    Factor out $h$:

    $ h(\sqrt{3} + 1) = 200 $

    Isolate $h$:

    $ h = \frac{200}{\sqrt{3} + 1} $

  6. Rationalize the Denominator:

    Multiply by the conjugate $(\sqrt{3} - 1)$:

    $ h = \frac{200(\sqrt{3} - 1)}{(\sqrt{3} + 1)(\sqrt{3} - 1)} = \frac{200(\sqrt{3} - 1)}{3 - 1} $

    $ h = \frac{200(\sqrt{3} - 1)}{2} $

    $ h = 100(\sqrt{3} - 1)\text{ m} $

The height of the cliff is $100(\sqrt{3} - 1)\text{ m}$.

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Important Questions from Heights and Distances

  1. If x is the distance of P from the bottom of the pillar, then consider the following statements :

    1. x can take two values which are in the ratio 1 : 3

    2. x can be equal to the height of the flagstaff

    Which of the statements given above is/are correct?

  2. What is a possible value of tan θ ? 

  3. A vertical tower standing on a levelled field is mounted with a vertical flag staff of length 3 m. From a point on the field, the angles of elevation of the bottom and tip of the flag staff are 30° and 45° respectively. Which one of the following gives the best approximation to the height of the tower?

  4. Two poles are 10 m and 20 m high. The line joining their tops makes an angle of 15° with the horizontal. The distance between the poles is approximately equal to

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