The PMBOK lists for EAC:

EAC = AC + Bottoms Up ETC (per PMBOK, this is the most common in real life) Use if initial estimate is flawed

EAC = BAC/CPI Use if you are continuing with the same cost efficiency (any variances will continue) Per the internet, this is the most common formula used for test answers

EAC = AC + (BAC - EV) Use if any variances you have experienced will not occur again, and you will continue at the planned aka budgeted rate

EAC = AC ((BAC=EV)/CPI * SPI)) Use if you have a schedule goal or constraint.

OK on the internet, several people have also mentioned, EAC = AC + (BAC-EV)/CPI. What is the real purpose of this? Its not in the PMBOK (not saying it is not right, im just a bit confused)

The PMBOK lists for ETC:

ETC = EAC - AC

Reestimate

Thats it! I have also seen ETC = BAC - EV. This would make sense if you are continuing at the budgeted rate. Yet I have also seen ETC = (BAC - EV)/CPI, what is the purpose of this formula?

I assuming that PMBOK only lists the ETC = EAC - AC formula b/c you can use this formula for any situation. Just calculate the EAC based on the situation (so if your at the budgeted rate or any variance was will not occur again, use EAC = AC + (BAC-EV), at the cost efficiency rate and variances will continue then use EAC = BAC/CPI, etc).   Is this a correct assumption?