TOPIC: EV EXAM QUESTION

Hi,
I got a regular email from PMP Exam Formulas.
This one was on Earned Value.
I am confused by what seem like 2 contradictory statements:
For ETC, it says:
ETC = EAC-AC
This formula is used when current EVM figures are thought to be valid and can be used to predict future performance.
But then for EAC it says:
EAC = AC+ETC
The original estimate was fundamentally flawed or conditions have changed and invalidated original estimating assumptions.
Whats the right scenario to watch out for in the exam?