Calculates probabilities for a negative binomial distribution.
NEGBINOMDIST(x, r, p)
For independent trials each with a probability p of success, NEGBINOMDIST returns the probability that there will be exactly x failures before there have been r successes. The formula used is:
NEGBINOMDIST(1, 1, 0.5)
returns 0.25 (25%), the probability that heads will come up exactly once before tails has come up when tossing a coin.