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1l. Suppose that X1, X2,... Xn are independent random variables. Assume that ElXi] /4 and Var(X )-σ, where i 1, 2, . .., n. If ai , aam. , an are constants. 1,a2, , an are constan (i) Write down expression for (i) E{Σ,i ai Xi) and (ii) Var(Li la(Xi). (i) Rewrite the expression if X,s are not independent.
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Answer #1

(i)

diMi に! に! に!

Var( 〉 aix, a, a,Cov(X., Xi i,j-1 i,j-1

X1 , X2 , .... , Xn are independent random variables Rightarrow Cov(Xi , Xj) = 0 v i,j=1,2,...,n ; ieqj

aVar(X)+0 i=1 i=1 i=1

(ii)

Var( 〉 aix, a, a,Cov(X., Xi i,j-1 i,j-1

X1 , X2 , .... , Xn are not independent random variables.

Let ho ij be the correlation between Xi and Xj  v i,j=1,2,...,n ; ieqj

herefore Cov(Xi , Xj) = ho ijsigmaisigmajv i,j=1,2,...,n ; ieqj

i,j-1 i,j-1

diMi に! に! に!

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explan the answer 1l. Suppose that X1, X2,... Xn are independent random variables. Assume that ElXi]...
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