• Sep 29th 2012, 11:50 PM
MartinDalskov
Hey!

I have some variance problems, that I hope you can help me solving - here we go:

"Given that var X = var Y = cov (X,Y) = 1, find
a) Var(3-X)
b) Var (2X+4)
..."

I just want to get started. I've checked all of the statistics book, that we have at college, and it really won't be solved ;) The answer to these to problems are:

"a) 1
b) 4"

I just want to now which formula, I should use - I have look everywhere now :(

• Sep 29th 2012, 11:54 PM
chiro
Hey MartinDalskov.

For these expressions, there is a well known one for variance and with two variables we have:

Var[X+Y] = Var[X] + Var[Y] + 2*Cov(X,Y) with
Var[aX] = a^2*Var[X] and
Var[X+c] = Var[X] for some constant c.
If X and Y are independent then Cov(X,Y) = 0 but if not you must include this term.
• Sep 30th 2012, 12:02 AM
MartinDalskov
Hi again.

Yes, I've tried that for about two hours now - Both way around. It just won't give 1 ;) Not in my world ;)

Could you insert it, in the correct order for me? Just the a one?
• Sep 30th 2012, 12:03 AM
chiro
Recall Var[-X] = (-1)^2Var[X] = Var[X].

Var[3-X] = Var[-X] = Var[X].

The second one is very similar (using the same kind of rules).
• Sep 30th 2012, 12:06 AM
MartinDalskov