Depression to tower top and bottom from a cliff: From the top of a cliff of height h metres, the angles of depression of the top and the bottom of a nearby vertical tower are 30° and 60° respectively. Find the height of the tower in terms of h.

Difficulty: Medium

Correct Answer: 2h/3

Explanation:


Introduction / Context:
Two angles of depression from a higher point to two points on or above the ground allow expressing horizontal distance and relative verticals, yielding the unknown tower height as a function of h.


Given Data / Assumptions:

  • Cliff height = h.
  • Angles of depression: 30° to tower top, 60° to tower bottom.
  • Tower is vertical; ground is level.


Concept / Approach:
If horizontal distance from cliff foot to tower base is x, then tan 60° = h / x → x = h/√3. For the top, vertical drop is h - T (T = tower height). tan 30° = (h - T)/x → solve for T.


Step-by-Step Solution:

x = h / √3 from tan 60°.tan 30° = (h - T)/x = 1/√3.Thus h - T = x/√3 = (h/√3)/√3 = h/3.So T = h - h/3 = 2h/3.


Verification / Alternative check:
Insert T back: Top drop = h - 2h/3 = h/3 → tan 30° = (h/3)/(h/√3) = 1/√3 ✔️.


Why Other Options Are Wrong:
h√3, 2h√3, h/3 mismatch the tangent relations; only 2h/3 satisfies both angles simultaneously.


Common Pitfalls:
Confusing elevation with depression; misplacing T in h − T; forgetting that both rays share the same horizontal distance x.


Final Answer:
2h/3

More Questions from Height and Distance

Discussion & Comments

No comments yet. Be the first to comment!
Join Discussion