The WEIBULL function returns the Weibull distribution. If the cumulative boolean is true it will return: 1 - exp (-(x/beta)^alpha), otherwise it will return (alpha/beta^alpha) * x^(alpha-1) * exp(-(x/beta^alpha)).
if x < 0 WEIBULL returns #NUM! error. if alpha <= 0 or beta <= 0 WEIBULL returns #NUM! error.