Generating function of binomial coefficients We want to evaluate the sum $$\sum_{L=0}^{\infty}\frac{1}{2}L(L+1)x^L$$
From this set of notes (page 2, equation 8) we find the formula $$\sum_{n=0}^{\infty}\binom{n}{k}y^n = \frac{y^n}{(1-y)^{n+1}}$$ which suggests that I can do $$\frac{1}{x}\sum_{L=0}^{\infty}\binom{L+1}{2}x^{L+1} = \frac{x^L}{(1-x)^{L+2}}\tag{1}\label{eqn1}$$ since $$\frac{1}{2}L(L+1) = \frac{(L+1)!}{2!((L+1)-2)!}$$
But if I do 
\begin{aligned}
\sum_{L=0}^{\infty}\frac{1}{2}L(L+1)x^L & = \sum_{L=0}^{\infty}\frac{L}{2}\frac{d}{dx}x^{L+1} \\ 
& = \sum_{L=0}^{\infty} \frac{1}{2} \frac{d}{dx} \left[(L+2)x^{L+1} - 2x^{L+1}\right] \\
& = \frac{1}{2} \frac{d^2}{dx^2} \sum_{L=0}^{\infty} x^{L+2} - \frac{d}{dx} \sum_{L=0}^{\infty} x^{L+1} \\
& = \frac{1}{2} \frac{d^2}{dx^2} \frac{x^2}{1-x} - \frac{d}{dx} \frac{x}{1-x} \\
& = \frac{1}{2} \frac{d}{dx} \left[2x(1-x)^{-1} + x^2(1-x)^{-2}\right] - \left[(1-x)^{-1} + x(1-x)^{-2}\right] \\
& = \frac{1}{2} \frac{d}{dx} \frac{2x-x^2}{(1-x)^2} - \frac{1}{(1-x)^2} \\
& = \frac{1}{2} \frac{d}{dx} \left[\frac{x}{(1-x)^2} + \frac{x}{1-x} \right] - \frac{1}{(1-x)^2} \\
& = \frac{1}{2} \left[(1-x)^{-2} + 2x(1-x)^{-3} \right] \\
& = \frac{1+x}{2(1-x)^3}
\end{aligned}
which is nowhere close to (1). Where did I go wrong?
All help is welcome.

Note:   The cited formula (8) is not correct. There is a typo since we have
\begin{align*}
\sum_{n=0}^{\infty}\binom{n}{k}y^n = \frac{y^{\color{blue}{k}}}{(1-y)^{\color{blue}{k+1}}}\tag{1}
\end{align*}
We  therefore   expect
\begin{align*}
\frac{1}{x}\sum_{L=0}^{\infty}\binom{L+1}{2}x^{L+1} =\frac{1}{x}\sum_{L=1}^{\infty}\binom{L}{2}x^L
= \frac{x^{\color{blue}{1}}}{(1-x)^{\color{blue}{3}}}\tag{2}
\end{align*}

Your derivation is fine  up to
\begin{align*}
 \frac{d}{dx}& \frac{2x-x^2}{(1-x)^2} - \frac{1}{(1-x)^2} \\
&=\frac{1}{2}\left((2-2x)(1-x)^{-2}+(2x-x^2)(+2)(1-x)^{-3}\right)-\frac{1}{(1-x)^2}\\
&=\left(\frac{1}{1-x}+\frac{2x-x^2}{(1-x)^3}\right)-\frac{1}{(1-x)^2}\\
&=\frac{1}{(1-x)^3}-\frac{1}{(1-x)^2}\\
&\,\,\color{blue}{=\frac{x}{(1-x)^3}}
\end{align*}
in  accordance with (2).

Hint:  With respect  to  (1)   we   have
\begin{align*}
\frac{y^k}{(1-y)^{k+1}}&=y^k\sum_{n=0}^\infty \binom{-(k+1)}{n}(-y)^n\tag{3}\\
&=\sum_{n=0}^\infty \binom{k+n}{n}y^{n+k}\tag{4}\\
&=\sum_{n=k}^\infty \binom{n}{n-k}y^n=\sum_{n=k}^\infty\binom{n}{k}y^n\tag{5}\\
&=\sum_{n=0}^\infty\binom{n}{k}y^n\tag{6}
\end{align*}
Comment:


*

*In (3) we use the binomial series expansion.

*In (4) we use the binomial identity $\binom{-p}{q}=\binom{p+q-1}{q}(-1)^q$.

*In (5) we shift the index to start with $n=k$.

*In (6) we use $\binom{p}{q}=0$ if $q>p$.
