BETAINV(p,alpha,beta,a,b)
p: probability
alpha: scale parameter
beta: scale parameter
a: optional lower bound, defaults to 0
b: optional upper bound, defaults to 1
If p < 0 or p > 1 this function returns a #NUM! error. If alpha <= 0 or beta <= 0, this function returns a #NUM! error. If a >= b this function returns a #NUM! error.
This function is Excel compatible.