Declaration: 
FDistriIntegral (F: double; df1, df2: integer): double; 
The function FDistriIntegral returns the integral of the Fdistribution between zero and the value of the parameter F. The parameters df1 and df2 define the degrees of freedom and must be greater or equal to 1. The returned value may vary between 0.0 and 1.0.

Hint 1: 
The calculated value is exact to about 8 decimal places. 
Hint 2: 
The function FDistriIntegral calculates the integral from 0.0 to the F value. This differs from the convention of how the integral of the F distribution is used in many statistical packages, which tabulate the integral between +infinity and the F value. Thus you have to use 1FDistriIntegral to get comparable results. For example, in order to calculate the function FDIST(x,df1,df2) of Microsofts Excel® you have to use 1FDistriIntegral(x,df1,df2). 
