## Precalculus (6th Edition) Blitzer

If it is impossible for events $A\ \text{ and }\ B$ to occur simultaneously, the events are said to be mutually exclusive. For such events $P\left( A\ \text{or}\ B \right)=P\left( A \right)+P\left( B \right)$.
We know that if it is impossible for any two events, $A\ \text{ and }\ B$ to occur simultaneously, they are said to be mutually exclusive. If $A\ \text{ and }\ B$ are mutually exclusive events, the probability that either $A\ \text{or }B$ will occur is determined by adding their individual probabilities. Thus, the probability is: $P\left( A\ \text{or}\ B \right)=P\left( A \right)+P\left( B \right)$