Hi!

I have problems with figuring out, how to get from the following expression:

$\displaystyle \sum_{k=n}^{t} {k-1\choose n-1}p^{n}(1-p)^{k-n} - \sum_{k=n+1}^{t} {k-1\choose n}p^{n+1}(1-p)^{k-n-1} $

to that one:

$\displaystyle {t\choose n}p^{n}(1-p)^{t-n} $

Thanks in advance for any help.