S1 - Discrete Random Variables - Rep Available

hello,

I was wondering if I was doing this question right...

2) The probability distribution for the random variable X is given by:

P(X=x) = ax for x = 1,2,3,4,5,6,7,8

a) Find a
b) Calculate E(X)
c) Calculate Var(X)

a I knew the total of x was 36, and the probability must add up to 1, so a must = 1/36

b I used the formula ( 1 x 1/36 )+( 2 x 2/36 ) ... + (8 x 8/36)
to get 17/3

c I used the formula above, but I just squared the x value to get 36

I then found the varient by doing 36 - (17/3)^2 = 35/9

Am i doing this correctly??

Thanks

Reply 1

Daniel Freedman

That's the way you should do it (I haven't checked your calculations though)

Reply 2

The Young Economist

Seems fine to me sir!...of the top of my head though

Reply 3

roosel4

Awesome, thanks both of you!

Reply 4

DarkWhite

a and b seem fine.

I'm not sure what formula you're using for Variance. Var(X) = E(X^2) – E(X)^2 (mean of the squares minus the square of the mean). Yours could be right too, but I'm in a rush so haven't got too much time to read!