Calculates the inverse of the BETADIST function.
BETAINV(p, α, β, a, b)
The beta distribution is a family of continuous probability distributions, defined for an interval a to b, where α and β are parameters controlling the shape of the distribution.
BETAINV returns the value n, lying between a and b, such that BETADIST(n, α, β, a, b) is p.
a and b are optional parameters which default (if omitted) to 0 and 1.
BETAINV(0.96, 3, 4)
returns approximately 0.75.