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# S1 help Tweet

Maths and statistics discussion, revision, exam and homework help.

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1. S1 help
Well they give the mean as 2 and the variance as 9.

I am meant to find the answer of E[(y-1)(y+1)]
how do i do it?
2. Re: S1 help
Try expanding the bracket inside and then think about how you can use what has been given to you to work out the answer.
3. Re: S1 help
If you're still having trouble, here are more hints:

Spoiler:
Show
Mean of X = E[X]
Variance of X = E[X^2]-E[X]^2
Mean of X+c = E[X+c] = E[X]+c
Last edited by aznkid66; 27-06-2012 at 19:35.
4. Re: S1 help
Still can't get it
5. Re: S1 help
(Original post by bigboy1111452)
Well they give the mean as 2 and the variance as 9.

I am meant to find the answer of E[(y-1)(y+1)]
how do i do it?

edit: ignore that wasn't thinking that 'y' is the distribution
Last edited by natninja; 27-06-2012 at 20:45.
6. Re: S1 help
E[(y+1)(y-1)] = E(Y2-1) = E(Y2)-1

Now, what is E(Y2)? Think about the relationship between E(Y2), E(Y) and Var(Y).
7. Re: S1 help
More helpful versions of the equations:

Spoiler:
Show
Mean of X = E[X]
Mean of Y = E[Y] = 2

Variance of X = E[X2]-E[X]2
Variance of Y = E[Y2]-E[Y]2=E[Y2]-22 = 9
Therefore: Mean of Y2 = ...

E[X+c] = E[X] + c
E[(Y-1)(Y+1)] = E[Y2-1] = E[Y2]-1