Calculates values for a Student's t-distribution.
TDIST(x, r, mode)
x is the value at which the distribution is calculated.
r, an integer >=1, is the degrees of freedom parameter for the t-distribution.
mode may be 1 or 2, to calculate the one or two tailed probability.
TDIST returns the probability density function for the t-distribution, calculating:
TDIST(0.5, 2, 2)
returns approximately 0.666.