expectation

**blueturkey** · May 8th 2010, 10:17 AM

Hi, I've got this problem with the solutions but I dont understand how to get from line 1 to line 2

I've attached the solution.

Im not sure why the sum to infinity of x.lamda^x results in lambda.e^lambda

Thanks in advance for any help clarifying.

**Anonymous1** · May 8th 2010, 10:25 AM

Originally Posted by blueturkey

Hi, I've got this problem with the solutions but I dont understand how to get from line 1 to line 2

I've attached the solution.

Im not sure why the sum to infinity of x.lamda^x results in lambda.e^lambda

Thanks in advance for any help clarifying.

$\sum x[\frac{1}{2}\frac{e^{-\lambda}\lambda^x}{x!} + \frac{1}{2}\frac{e^{-\mu}\mu^x}{x!}]$

$= \sum [x\frac{1}{2}\frac{e^{-\lambda}\lambda^x}{x!} + x\frac{1}{2}\frac{e^{-\mu}\mu^x}{x!}]$

$= \sum x\frac{1}{2}\frac{e^{-\lambda}\lambda^x}{x!} + \sum x\frac{1}{2}\frac{e^{-\mu}\mu^x}{x!}$

$= \frac{e^{-\lambda}}{2}\sum \frac{x\lambda^x}{x!} + \frac{e^{-\mu}}{2}\sum\frac{x\mu^x}{x!}$

**blueturkey** · May 8th 2010, 10:44 AM

Many thanks! that was very clear.

May I also ask how to get from line 2 to line 3? I know that it is partly due to the taylors series expansion but I'm a bit confused as to where the x went in the third line.

**Anonymous1** · May 8th 2010, 10:58 AM

Originally Posted by blueturkey

Many thanks! that was very clear.

May I also ask how to get from line 2 to line 3? I know that it is partly due to the taylors series expansion but I'm a bit confused as to where the x went in the third line.

The well known Taylor expansion is:

$\sum \frac{\lambda^x}{x!} = \underbrace{(1 + \lambda + \frac{\lambda^2}{2!} +...)} = e^{\lambda}$

Your sum is:

$\sum \frac{x\lambda^x}{x!} = 0 + \lambda + 2\frac{\lambda^2}{2!} + 3\frac{\lambda^3}{3!}+... = \lambda(\frac{\lambda}{\lambda} + 2\frac{\lambda^2}{\lambda 2!} +3\frac{\lambda^3}{\lambda 3!}+...) = \lambda\underbrace{(1 + \lambda + \frac{\lambda^2}{2!} +...)} = \lambda e^{\lambda}$

Is this clear?

**blueturkey** · May 8th 2010, 12:46 PM

Yes, it is, thank you very much, this expansion has been very useful.

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