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NDA 2025
Exam
NDA
Year
2025
Subject
Mathematics
Chapter
Heights and Distances

NDA 2025 · Mathematics · Heights and Distances

Consider the following for the two (02) items that follow: The top...

Consider the following for the two (02) items that follow:
The top (M)ofatowerisobservedfromthreepoints) of a tower is observed from three pointsP,,QandandRlyinginahorizontalstraightlinewhichpassesdirectlyalongthefoot(Nlying in a horizontal straight line which passes directly along the foot (N) of the tower. The angles of elevations of MM from PP, QQ and RR are 30°30°, 45°45° and 60°60° respectively. Let PQ=aPQ = a and QR=bQR = b.

What is MNMN equal to?

Options

  1. A(3+32)b\left(\dfrac{3+\sqrt{3}}{2}\right)b
  2. B(3−32)b\left(\dfrac{3-\sqrt{3}}{2}\right)b
  3. C(3−34)b\left(\dfrac{3-\sqrt{3}}{4}\right)b
  4. D(3+34)b\left(\dfrac{3+\sqrt{3}}{4}\right)b

Correct answer

A. (3+32)b\left(\dfrac{3+\sqrt{3}}{2}\right)b

Step-by-step solution

Let MN=hMN = h be the height of the tower.

The distances of points PP, QQ, and RR from the foot of the tower NN are given by:

PN=hcot⁡30∘=h3PN = h \cot 30^{{}^{\circ}} = h\sqrt{3}

QN=hcot⁡45∘=hQN = h \cot 45^{{}^{\circ}} = h

RN=hcot⁡60∘=h3RN = h \cot 60^{{}^{\circ}} = \dfrac{h}{\sqrt{3}}

Given that PP, QQ, RR lie on a straight line passing through NN, the distance QRQR is:

QR=QN−RNQR = QN - RN

b=h−h3b = h - \dfrac{h}{\sqrt{3}}

b=h(3−13)<br><br>b = h \left(\dfrac{\sqrt{3}-1}{\sqrt{3}}\right)<br><br>h = b33−1<br><br>Rationalizing\dfrac{b\sqrt{3}}{\sqrt{3}-1}<br><br>Rationalizing the denominator:

h = b3(3+1)(3−1)(3+1)<br><br>\dfrac{b\sqrt{3}(\sqrt{3}+1)}{(\sqrt{3}-1)(\sqrt{3}+1)}<br><br>h = b(3+3)3−1<br><br>\dfrac{b(3+\sqrt{3})}{3-1}<br><br>h = \left(\dfrac{3+\sqrt{3}}{2}\right)bb

Answer: \left(\dfrac{3+\sqrt{3}}{2}\right)b
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