# What do the variables in the Negative Binomial Distribution mean?

So far what i know is that r = success, x = trials, x-r = failures, p = probability of success, q = failure. I am confused whether these are correct.

$P(x=1) ={ \binom{x+r-1}{r-1} ( p^r ) (1-p)^x}$

In your case $r=$success,$x=$trials, $x-r=$failures and $p=$success prob.
So you have $r$ successes and $x-r$ failures and wish to measure the probability for the number of trials required:
$$P(X=x) = \binom{r+(x-r)-1}{r}(p^r)(1-p)^{x-r}$$