which one of these formulas should you use in which situation:
Var(aX+bY)=a^2Var(X) + b^2Var(Y)
Var(aX+bY)=aVar(X) + bVar(Y)

Also, which applies to what when taking away variances?

Question

which one of these formulas should you use in which situation:
Var(aX+bY)=a^2Var(X) + b^2Var(Y)
Var(aX+bY)=aVar(X) + bVar(Y)

Also, which applies to what when taking away variances?

anonymous · Answer

@epoweritheta

epoweritheta · Answer

oh...sorry i dont know stat

anonymous · Answer

no worries :)

anonymous · Answer

use the first formula when the co-variance between x and y is 0

anonymous · Answer

thanks :) and so i assume the other is used when its not equal to 0?

anonymous · Answer

i don't know of any situation for the formula 2 to be used, except for a,b=0,1

anonymous · Answer

in the mark scheme for the question i was stuck on they had used the second formula.
 x~n(4.4,0.2^2)      y~n(2.6,0.15^2)

find P(2x + 3y > 16)