F_DIST()

Calculates the F Probability Distribution.                        
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Passed the F value, the degrees of freedom for the numerator and  
the degrees of freedom for the denominator.    The F distribution 
is a natural extension of the chi-square  distribution because the
F distribution is simply a ratio of two  chi-square variables.    
The F distribution usually is concerned  with relationships       
between two or more variables such as Analysis  of Variance       
(ANOVA).