1. Bayes' theorem is used to calculate unknown probabilities based on additional information provided by experiments or past records. It involves calculating inverse or posterior probabilities. 2. The theorem states that the probability of an event occurring given some evidence is equal to the probability of the evidence given the event occurred, multiplied by the original probability of the event, divided by the total probability of the evidence. 3. Bayes' theorem is widely used in business and engineering to help decision makers account for uncertainty in their analyses and decisions. It allows probabilities to be updated as new information becomes available.