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GATE 2020 ST – Question 42

Joint Distributions · Joint and Conditional Distributions · 2 marks · Multiple choice

On[0,2π]³ the joint density is $(1-\sin x_1\sin x_2\sin x_3)/(8\pi^3)$, zero elsewhere. Which is true?

  1. The three variables are mutually independent.
  2. They are pairwise independent.
  3. (X₁,X₂) is independent of X₃.
  4. Var(X₁+X₂)=π².

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Show answer and explanation

Correct answer: (B) They are pairwise independent.

Explanation

Integrating over any coordinate kills the sine term because its integral over a full period is zero. Every pair therefore has constant density1/(4π²), the product of two uniform marginal densities. Thus pairwise independence holds, B. The full joint sine term prevents mutual independence; Var(X₁+X₂)=2π²/3, notπ².

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