Calculates the inverse of the two-tailed TDIST function.
returns the value x, such that TDIST(x, r, 2) is p.
r (degrees of freedom) is an integer >=1.
p (probability) must be >=0 and <=1.
To calculate the inverse of the one-tailed TDIST function, use TINV(p*2, r), which returns the value x, such that TDIST(x, r, 1) is p.
returns approximately 0.50.