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GATE 2020 ST – Question 64

Joint Distributions · Order Statistics · 2 marks · Numerical answer

Joint density is $(81/4)x_1^2x_2^2x_3^2$ on−1≤x₁≤x₂≤x₃≤1, zero otherwise. Find Var(X₁+X₂+X₃) (one decimal).

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Correct answer: 1.8 to 1.8

Explanation

This is the ordered-sample density for three iid variables with base density g(x)=3x²/2 on[−1,1], since3!×(3/2)³=81/4. Ordering does not alter their sum. The base variable has mean0 and second moment(3/2)∫₋₁¹x⁴dx=3/5. Independence before ordering gives sum variance3×3/5=1.8.

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