# 3.5.3.1.1 Betacdf

## Description

Computes beta cumulative distribution function at x, with parameters a and b. The pair of parameters a and b must all be positive, and the values in x must lie on the interval [0,1]. The parameter tail determines the returned probability is the lower tailed or upper tailed.

## Syntax

double prob = betacdf(double x,  double a,  double b[, int tail])

## Parameters

x

Input, the value of the x variate, must lie on the interval [0, 1].

a

Input, the first shape parameter, a, of the required beta distribution, must be positive( a>0 ).

b

Input, the second shape parameter, b, of the required beta distribution, must be positive ( b>0 ).

tail

Input, Optional. The tail probability type using for the beta distribution
• 1 = the for upper tail probability
• 2 = the lower tail probability (Default if not set)

prob

Output. The probability that a single observation from a beta distribution with parameters a and b will fall in the interval [0, x] for upper tailed and [x, 1] for the lower tailed.

## Example

aa = betacdf(1, 2, 2);
aa = ; //aa=1
betacdf(0.9,2,25,1)=; //betacdf(0.9,2,25,1)=2.35E-24