Answer:  The third axiom of probability.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Question:  What is, "For any countable collection A1, A2, ... of mutually exclusive events,  P[Èi=1,2,... Ai] = P[A1] + P[A2] + ..."?