Question:

The domain of the function
\(f(x) = \sin^{-1}\left(\frac{|x| + 5}{x^2 + 1}\right)\)
is \((-\infty, -a] \cup [a, \infty)\). Then a is equal to :

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Need \(\frac{|x|+5}{x^2+1}\le 1\) (it is always positive); put \(t=|x|\) and solve \(t^2 - t - 4 \ge 0\).
Updated On: Nov 5, 2026
  • \(\frac{\sqrt{17}-1}{2}\)
  • \(\frac{1+\sqrt{17}}{2}\)
  • \(\frac{\sqrt{17}}{2}+1\)
  • \(\frac{\sqrt{17}}{2}\)
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The Correct Option is B

Solution and Explanation

For \(\sin^{-1}\) to be defined, \(-1\le \frac{|x|+5}{x^2+1}\le 1\). The fraction is always positive, so only \(\frac{|x|+5}{x^2+1}\le1\) matters.
\(|x| + 5 \le x^2 + 1 \Rightarrow |x|^2 - |x| - 4 \ge 0\).
Roots of \(t^2 - t - 4 = 0\): \(t = \frac{1\pm\sqrt{17}}{2}\). Since \(t=|x|\ge0\), we need \(|x| \ge \frac{1+\sqrt{17}}{2}\).
So the domain is \((-\infty,-a]\cup[a,\infty)\) with \(a = \frac{1+\sqrt{17}}{2}\).
Answer: (2) \(\frac{1+\sqrt{17}}{2}\)
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