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Suppose $X$ has a $Binomial(n,p)$ distribution. Then its moment generating function is

$$ M(t) = \sum_{x=0}^x {n \choose x}p^x(1-p)^{n-x} \\ =\sum_{x=0}^{n} {n \choose x}(pe^t)^x(1-p)^{n-x}\\ =(pe^t+1-p)^n $$

Can someone please explain how the sum is obtained from lines (2) to (3)?

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This is the Binomial formula. – Stefan Hansen Nov 13 '12 at 19:53
It makes sense to me that the Binomial Theorem would be applied to this, I'm just having a hard time working out how they get to the final result using it :\ – TopGunCpp Nov 13 '12 at 21:02
It all makes sense now, it "is" a syntactically simplified way to write the Binomial Theorem. Thanks for the clarification – TopGunCpp Nov 13 '12 at 21:07
Call $l=pe^t$ and $j=1-p$, then the second line is $\sum_{x=0}^n {n \choose x} l^x j^{n-x} = (l+j)^n$ by the binomial formula. – Stefan Hansen Nov 13 '12 at 21:07

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