PERT : Triangular Distribution Vs Beta Distribution

PMBOK 5th Edition (Pg 206) gives the below 2 formulae for PERT calculations - 

Triangular Distribution : cE = (cO + cM + cP) / 3

Beta Distribution : cE = (cO + 4cM + cP) / 6 

In which question scenarios do you use each of the 2 to find the cost estimates

 

I have given a few mock exams, but never have I come across a question where the correct answer was derived with the Triangular distribution formulae. 

 

In case if you have come across a qs needing Triangular Distribution, please let me know the qs. 

 

TIA.