prove an identity involving beta function and gamma function We know that $B(p,q)=\Gamma(p)\Gamma(q)/\Gamma(p+q)$ where $p, q>0$,
and $B(p,q)$ is related to binomial coefficients if one of $p,q$ is an integer. I want to prove the following identity.
$$\frac{\Gamma(b)}{\Gamma(b+2n+3)}\sum_{k=0}^n \frac{(-1)^{k+1}\Gamma(b+n+k+2)}{(n-k+1)!(k+1)!\Gamma(b+k+1)}=\frac{1}{(n+1)!(n+2)!}\left[(-1)^{n+1}B(b,n+2)-B(b+n+1,n+2)\right],$$
where all variables except $b$ are nonnegative integers, and $b=p/2+q$ for nonnegative integers $p,q.$ In fact, the value of $b$ is not essential here, as long as $b\geq 0$.
I tried to convert the above expression into an identity about binomial coefficients but still could not figure it out. 
Note that for the special case where $n=0$, it can be verified that the above identity holds. 
 A: Suppose we are interested in the value of
$$S_b(n) = 
\sum_{k=0}^n 
\frac{(-1)^{k+1}}{(n-k+1)!(k+1)!}
\frac{\Gamma(b+n+k+2)}{\Gamma(b+k+1)}.$$
This is
$$\sum_{k=0}^n 
\frac{(-1)^{k+1}}{(n-k+1)!(k+1)!}
(n+1)! {b+n+k+1\choose n+1}
\\ = \frac{1}{n+2} \sum_{k=0}^n 
\frac{(-1)^{k+1} (n+2)!}{(n-k+1)!(k+1)!}
{b+n+k+1\choose n+1}
\\ = \frac{1}{n+2} \sum_{k=0}^n (-1)^{k+1}
{n+2\choose k+1} {b+n+k+1\choose n+1}
\\ = \frac{1}{n+2} \sum_{k=1}^{n+1} (-1)^k
{n+2\choose k} {b+n+k\choose n+1}.$$
This becomes
$$-\frac{1}{n+2} {n+b\choose n+1}
- \frac{1}{n+2} (-1)^n {2n+b+2\choose n+1}
\\ + \frac{1}{n+2} \sum_{k=0}^{n+2} (-1)^k
{n+2\choose k} {b+n+k\choose n+1}.$$
To evaluate the sum we introduce
$${b+n+k\choose n+1}
= \frac{1}{2\pi i}
\int_{|z|=\epsilon}
\frac{(1+z)^{b+n+k}}{z^{n+2}} \; dz.$$
This yields for the sum
$$\frac{1}{2\pi i}
\int_{|z|=\epsilon}
\frac{(1+z)^{b+n}}{z^{n+2}}
\sum_{k=0}^{n+2} (-1)^k {n+2\choose k} (1+z)^{k} \; dz
\\ = \frac{(-1)^{n+2}}{2\pi i}
\int_{|z|=\epsilon}
\frac{(1+z)^{b+n}}{z^{n+2}}
(-1+1+z)^{n+2} \; dz
\\ = \frac{(-1)^{n}}{2\pi i}
\int_{|z|=\epsilon}
(1+z)^{b+n} \; dz = 0.$$
It follows that
$$S_b(n) = 
-\frac{1}{n+2} {n+b\choose n+1}
- \frac{1}{n+2} (-1)^n {2n+b+2\choose n+1}.$$
Converting this  into the  gamma function format  is pure  algebra and
left as an exercise to the reader.

Remark. We use $$(1+z)^{b+n}  = \exp((b+n)\log(1+z))$$ when $b$ is
not  an integer  where  the branch  cut  of the  logarithm  is on  the
negative real axis so that the branch point is at $z=-1.$ This ensures
that we have analyticity in a disk enclosing the origin.
