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What is  \(\cot^{-1} 9 + \text{cosec}^{-1} \frac{\sqrt{41}}{4}\)  equal to?

This question was previously asked in
NDA 2 2025 GAT Question Paper (14-Sep-2025)
The correct answer is

\(\pi\)

We need to evaluate the expression \(\cot^{-1} 9 + \text{cosec}^{-1} \frac{\sqrt{41}}{4}\).

  1. First, let's understand the individual components:
    • The inverse cotangent, \(\cot^{-1} x\), gives us an angle whose cotangent is \(x\).
    • The inverse cosecant, \(\text{cosec}^{-1} y\), gives us an angle whose cosecant is \(y\).
  2. From the problem, let \(\theta = \cot^{-1} 9\). This implies \(\cot \theta = 9\), so we need to find \(\sin \theta\), since it will be used in the cosecant function. Thus, if \(\cot \theta = 9\), we can assume a right triangle where the adjacent side is 9 and the opposite side is 1. Using the Pythagorean theorem:
    • \(\text{Hypotenuse}^2 = 9^2 + 1^2 = 81 + 1 = 82\)
    • \(\text{Hypotenuse} = \sqrt{82}\)
    • Since \(\sin \theta = \frac{1}{\text{Hypotenuse}}\), we have \(\sin \theta = \frac{1}{\sqrt{82}}\)
  3. Next consider \(\text{cosec}^{-1} \frac{\sqrt{41}}{4}\). This gives an angle \(\phi\) such that \(\text{cosec} \, \phi = \frac{\sqrt{41}}{4}\) meaning \(\sin \phi = \frac{4}{\sqrt{41}}\).
  4. Now, since we have: \(\sin^2 \theta + \sin^2 \phi\) should equal 1, due to the properties of the supplementary angles.
    • Calculate \(\sin^{2} \theta = \frac{1}{82}\) and \(\sin^{2} \phi = \frac{16}{41}\).
    • Simplify using \(\sin^{2} \phi + \sin^{2} \theta = 1\) \(\frac{16}{41} + \frac{1}{82} = \frac{32 + 1}{82}=1\), confirming \(\theta + \phi= \pi\)as supplementary angles satisfy this property.

Thus, \(\cot^{-1} 9 + \text{cosec}^{-1} \frac{\sqrt{41}}{4} = \pi\).

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