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Question

If $\cot A = \sqrt{5}$, then find the value of $\frac{\csc^2 A - \sec^2 A}{\csc^2 A + \sec^2 A}$

This question was previously asked in
RRB NTPC 2024 CBT 1 Question Paper (28-Aug-2025) (Shift 3)
The correct answer is
$\frac{2}{3}$

Calculating Trigonometric Expression Value

We are given $\cot A = \sqrt{5}$ and need to find the value of the expression $\frac{\csc^2 A - \sec^2 A}{\csc^2 A + \sec^2 A}$.

Finding Key Trigonometric Values

First, let's find the values of $\csc^2 A$ and $\sec^2 A$ using trigonometric identities.

  • Using the identity $\csc^2 A = 1 + \cot^2 A$: $ \csc^2 A = 1 + (\sqrt{5})^2 = 1 + 5 = 6 $
  • We know that $\tan A = \frac{1}{\cot A}$. So, $\tan A = \frac{1}{\sqrt{5}}$. Using the identity $\sec^2 A = 1 + \tan^2 A$: $ \sec^2 A = 1 + \left(\frac{1}{\sqrt{5}}\right)^2 = 1 + \frac{1}{5} = \frac{5+1}{5} = \frac{6}{5} $

Substituting Values into the Expression

Now, substitute the calculated values of $\csc^2 A$ and $\sec^2 A$ into the given expression:

$ \frac{\csc^2 A - \sec^2 A}{\csc^2 A + \sec^2 A} = \frac{6 - \frac{6}{5}}{6 + \frac{6}{5}} $

Simplifying the Expression

Simplify the numerator and the denominator:

  • Numerator: $ 6 - \frac{6}{5} = \frac{30 - 6}{5} = \frac{24}{5} $
  • Denominator: $ 6 + \frac{6}{5} = \frac{30 + 6}{5} = \frac{36}{5} $

Now, divide the numerator by the denominator:

$ \frac{\frac{24}{5}}{\frac{36}{5}} = \frac{24}{5} \times \frac{5}{36} = \frac{24}{36} $

Simplify the fraction:

$ \frac{24}{36} = \frac{2 \times 12}{3 \times 12} = \frac{2}{3} $

Therefore, the value of the expression is $\frac{2}{3}$.

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