# Math Help - Expected Value

1. ## Expected Value

Let: X ~ Poisson( $\lambda$ )

Find: $E[X(X-1)]$
There is a certain method to go about doing this. I remember the start and finish, but not the intermediate steps.

$E[X^2 - X]$

$= E[X^2] - E[X]$

... ... ... ...

$= \lambda + \lambda ^2 - \lambda$

$= \lambda ^2$
Or something like it. It had to do with the fact that E[X] in this case = $\lambda$

2. Originally Posted by janvdl
There is a certain method to go about doing this. I remember the start and finish, but not the intermediate steps.

Or something like it. It had to do with the fact that E[X] in this case = $\lambda$
Hey there Jan!