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GATE 2021 BM – Question 11

Engineering Mathematics · Probability and Statistics · 1 mark · Multiple choice

For $f_X(x)=\frac1\pi\frac{q}{e^x+e^{-x}}$ to be a valid probability density on the real line, find q.

  1. 2
  2. $\pi$
  3. 4
  4. $-\pi$

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Show answer and explanation

Correct answer: (A) 2

Explanation

Let u=eˣ, so dx=du/u and 1/(eˣ+e⁻ˣ)dx=du/(1+u²). Integration over the real line becomes ∫₀∞du/(1+u²)=π/2. Thus the total probability is (q/π)(π/2)=q/2. Setting it to one gives q=2, A, also ensuring nonnegative density.

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