Computes cumulative distribution function at f, with parameters ndf, fdf, and tail. The parameters of ndf and fdf must all be positive, and the values in f must be positive. The parameter tail determines the returned probability is the lower tailed or upper tailed.
The result, prob, is the probability that a single observation from a distribution with parameters ndf and fdf will fall in the interval [0, f] for lower-tailed, [f, ] for upper-tailed.
double prob = fcdf(double f, double ndf, double fdf[, int tail])
fcdf(1000,20,20,1)=; //ANS: 9.0714457299284E-26