In reply to Rick Wicklin.

Thank you quite much for your patience!! What confuses me is actually the procedure leading to alpha's formula using estimate of minimum time(a),the most likely time(b) and the maximum time(c).

I have learnt another literature ——Teaching Project Simulation in Excel using PERT-Beta Distributions, written by Ron Davis, which gives complete formula for a Beta distribution defined on the interval [a,b] with parameters [Alpha,Beta,a,b].It has MEAN: mu = a + (b-a)*(Alpha/(Alpha+Beta) ;VARIANCE:(Alpha/(Alpha+Beta))*(Beta/(Alpha+Beta))*((b-a)2/(Alpha+Beta+1)).

However, I failed to derive "alpha = (mu-a)*(2*b-a-c)/((b-mu)*(c-a));" according to the formula above. Can you help me?