Let (X, Y) be BVN(0,0, 1, 1, p). Find the variance of XY.

A First Course in Probability (10th Edition)
10th Edition
ISBN:9780134753119
Author:Sheldon Ross
Publisher:Sheldon Ross
Chapter1: Combinatorial Analysis
Section: Chapter Questions
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Let (X,Y) be BVN(0,0, 1, 1, p). Find the variance of XY.
Transcribed Image Text:Let (X,Y) be BVN(0,0, 1, 1, p). Find the variance of XY.
Expert Solution
Step 1

Given that;

(X, Y)~BVN(0, 0, 1, 1, ρ)X~Normal(0,1) and Y~Normal(0,1)Y|X=x~Normal(ρx, 1-ρ2)

We know;

E(X2)=1, E(Y2)=1E(X4)=3, E(Y4)=3and;E(XY)=ρ

Now;

Var(Y|X=x)=1-ρ2E(Y2|X=x)-E2(Y|X=x)=1-ρ2E(Y2|X=x)-(ρx)2=1-ρ2E(Y2|X=x)=(ρx)2+1-ρ2

Hence;

E(X2Y2)=EE(X2Y2|X)=EX2E(Y2|X)=EX2(ρ2X2+1-ρ2)=ρ2E(X4)+(1-ρ2)E(X2)=3ρ2+1-ρ2=2ρ2+1

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