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Variance-Covariance Matrix

  • Thread starter cse63146
  • Start date
  • #1
452
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Homework Statement



Let [tex]\Sigma = [/tex]
( var(X1) cov(X1, X2) )
( cov (X2. X1) var(X2) )

Show that [tex]Var (a_1 X_1 + a_2 X_2) = a^T \Sigma a[/tex]

where [tex]a^T = [a_1 a_2][/tex] is the transpose of the of the column vector a

Homework Equations





The Attempt at a Solution



I got this far:

[tex]Var (a_1 X_1 + a_2 X_2) = a_1^2 Var(X_1) + a_2^2 Var(X_2) + 2a_1 a_2 Cov (X_1, X_2) = a_1^2 Var(X_1) + a_2^2 Var(X_2) + a_1 a_2 Cov (X_1, X_2) + a_1 a_2 Cov (X_2, X_1)[/tex]

Thats all I got so far, any hints
 

Answers and Replies

  • #2
392
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Aren't you done? Isn't that what [tex]a^T \Sigma a[/tex] is?
 
  • #3
452
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Thought there was more to it than that.

There's another part of the question that says: Using [tex]Var (a_1 X_1 + a_2 X_2)[/tex] show that for every choice of a1 and a2 that [tex]a^T \Sigma a \geq 0[/tex]

Can I assume that [tex]\Sigma[/tex] is always positive?
 
  • #4
392
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[tex]Var (a_1 X_1 + a_2 X_2)\ge0[/tex] always, since it's variance! And you just showed it equals [tex]a^T \Sigma a[/tex]
 

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